🐛 Common Errors Guide

IndentationError: unexpected indent

What it means: Python uses indentation to define code blocks. Your indentation is inconsistent — either you mixed tabs and spaces, or a line is indented when it shouldn't be.

# ❌ Broken
def greet():
    print("Hello")
      print("World")   # Extra indent!

# ✅ Fixed
def greet():
    print("Hello")
    print("World")     # Same indent level

Fix: Make sure every line in the same block has the same indentation. Use 4 spaces (not tabs). In VS Code, turn on "Render Whitespace" to see invisible characters.

NameError: name 'x' is not defined

What it means: You're using a variable that doesn't exist yet — either you misspelled it, forgot to create it, or it's out of scope.

# ❌ Broken
print(username)        # Never defined!

user_name = "Alice"
print(username)        # Misspelled! (underscore)

# ✅ Fixed
user_name = "Alice"
print(user_name)       # Matches exactly

Fix: Check the spelling carefully — Python is case-sensitive. Name and name are different variables.

TypeError: can only concatenate str (not "int") to str

What it means: You tried to combine a string and a number with +. Python won't auto-convert.

# ❌ Broken
age = 25
print("I am " + age + " years old")

# ✅ Fix 1: Convert to string
print("I am " + str(age) + " years old")

# ✅ Fix 2: Use f-string (better!)
print(f"I am {age} years old")

TypeError: 'str' object is not callable

What it means: You accidentally used a built-in function name as a variable, then tried to call it.

# ❌ Broken
list = [1, 2, 3]       # Overwrote the built-in list()!
new_list = list("abc") # Now list is a variable, not a function

input = "hello"        # Overwrote input()!
name = input("Name: ") # Crash!

# ✅ Fixed: Don't use built-in names as variables
my_list = [1, 2, 3]
user_input = "hello"

Fix: Never name variables list, dict, str, int, input, print, type, or id.

KeyError: 'name'

What it means: You tried to access a dictionary key that doesn't exist.

# ❌ Broken
user = {"email": "a@b.com"}
print(user["name"])        # Key doesn't exist!

# ✅ Fix 1: Use .get() with a default
print(user.get("name", "Unknown"))

# ✅ Fix 2: Check first
if "name" in user:
    print(user["name"])

IndexError: list index out of range

What it means: You tried to access an index that doesn't exist in the list.

# ❌ Broken
items = ["a", "b", "c"]
print(items[3])    # Only indices 0, 1, 2 exist!

# ✅ Fixed: Check length first
if len(items) > 3:
    print(items[3])

# Remember: last item is items[len(items) - 1] or items[-1]

ModuleNotFoundError: No module named 'requests'

What it means: You're importing a package that isn't installed.

# ❌ Error: import requests → ModuleNotFoundError

# ✅ Fix: Install it first
# pip install requests
# (or: pip install requests --break-system-packages)

Fix: Run pip install <package_name> in your terminal. Make sure you're using the same Python that your script runs with (python -m pip install requests is safest).

ReferenceError: x is not defined

What it means: Same as Python's NameError — the variable doesn't exist.

// ❌ Broken
console.log(userName);   // Never declared!

// ✅ Fixed
let userName = "Alice";
console.log(userName);

Common cause: Misspelling, or accessing a variable outside its let/const block scope.

TypeError: Cannot read properties of undefined (reading 'x')

What it means: You're trying to access a property on something that is undefined. This is the #1 JavaScript error.

// ❌ Broken
let user = undefined;
console.log(user.name);   // Can't read .name on undefined!

let data = { user: null };
console.log(data.user.name);  // Can't read .name on null!

// ✅ Fix 1: Optional chaining
console.log(data.user?.name);   // undefined (no crash)

// ✅ Fix 2: Check first
if (data.user) {
    console.log(data.user.name);
}

Fix: Use optional chaining (?.) or check that the object exists before accessing properties.

TypeError: x is not a function

What it means: You're trying to call something that isn't a function — often because you overwrote it or misspelled it.

// ❌ Broken
let greet = "hello";
greet();   // "hello" is a string, not a function!

// Also common: forgetting await on async functions
const data = fetch(url);    // data is a Promise, not the response!
data.json();                // TypeError!

// ✅ Fixed
const response = await fetch(url);  // await the Promise
const data = await response.json();

SyntaxError: Unexpected token

What it means: JavaScript found a character it didn't expect — usually a missing bracket, parenthesis, or comma.

// ❌ Missing closing brace
function greet() {
    console.log("hi");
// } ← forgot this!

// ❌ Missing comma in object
const user = {
    name: "Alice"
    email: "a@b.com"   // Missing comma after "Alice"!
};

// ✅ Fixed
const user = {
    name: "Alice",
    email: "a@b.com"
};

Fix: The error message tells you the line number. Look at that line and the line above for missing {, }, (, ), or ,.

== vs === (Silent Logic Bug)

What it means: Not an error message — but a constant source of bugs. == does type coercion; === does not.

// ❌ Surprising behavior with ==
"5" == 5       // true (string "5" coerced to number!)
0 == false     // true
"" == false    // true
null == undefined  // true

// ✅ Always use ===
"5" === 5      // false (different types)
0 === false    // false
"" === false   // false

Rule: Always use === and !== in JavaScript. There's almost never a reason to use ==.

SyntaxError: await is only valid in async functions

What it means: You used await outside an async function.

// ❌ Broken
function fetchData() {
    const response = await fetch(url);  // Can't use await here!
}

// ✅ Fixed: Add the async keyword
async function fetchData() {
    const response = await fetch(url);  // Now it works
}

CS0029: Cannot implicitly convert type 'string' to 'int'

What it means: C# is strictly typed — you can't assign a string to an int variable without explicit conversion.

// ❌ Broken
int age = Console.ReadLine();  // ReadLine returns string!

// ✅ Fixed
int age = int.Parse(Console.ReadLine()!);

// ✅ Safer: TryParse
if (int.TryParse(Console.ReadLine(), out int age))
    Console.WriteLine($"Age: {age}");
else
    Console.WriteLine("That's not a number!");

NullReferenceException: Object reference not set to an instance of an object

What it means: You tried to use a variable that is null. C#'s equivalent of JavaScript's "Cannot read properties of undefined."

// ❌ Broken
string? name = null;
Console.WriteLine(name.Length);  // Crash! name is null!

// ✅ Fix 1: Null check
if (name != null)
    Console.WriteLine(name.Length);

// ✅ Fix 2: Null-conditional operator
Console.WriteLine(name?.Length ?? 0);  // 0 if null

CS0103: The name 'x' does not exist in the current context

What it means: The variable is out of scope or was never declared.

// ❌ Broken: Variable declared inside if block
if (true)
{
    string message = "Hello";
}
Console.WriteLine(message);  // message doesn't exist here!

// ✅ Fixed: Declare in the outer scope
string message = "";
if (true)
{
    message = "Hello";
}
Console.WriteLine(message);

FormatException: Input string was not in a correct format

What it means: You called int.Parse() or double.Parse() on a string that isn't a valid number.

// ❌ Crashes if user types "abc"
int num = int.Parse(Console.ReadLine()!);

// ✅ Fixed: Use TryParse
if (int.TryParse(Console.ReadLine(), out int num))
    Console.WriteLine($"Number: {num}");
else
    Console.WriteLine("Invalid number!");

CS1503: Cannot convert from 'string' to 'char'

What it means: C# distinguishes between string (double quotes) and char (single quotes). Some methods need a char.

// ❌ Broken
string[] parts = text.Split(",");  // Split wants a char!

// ✅ Fixed: Use single quotes for char
string[] parts = text.Split(',');  // char ','

// Or use the string overload explicitly
string[] parts = text.Split(",", StringSplitOptions.None);

CS8600/CS8602: Possible null reference

What it means: The compiler warns that a value might be null and you're using it without checking. This is a warning, not an error, but ignoring it leads to runtime crashes.

// ⚠️ Warning: ReadLine() might return null
string name = Console.ReadLine();  // CS8600

// ✅ Fix 1: Null-forgiving operator (you know it won't be null)
string name = Console.ReadLine()!;

// ✅ Fix 2: Handle the null case
string? name = Console.ReadLine();
if (!string.IsNullOrEmpty(name))
    Console.WriteLine(name.ToUpper());

Off-By-One Error

What it is: Accessing one past the end of a collection, or looping one too many/few times.

# All languages: indices start at 0, not 1!
items = ["a", "b", "c"]   # indices: 0, 1, 2
# items[3] → ERROR (only 0-2 exist)
# Last item: items[len(items) - 1] or items[-1] (Python)

Rule: If a list has N items, valid indices are 0 through N-1.

Infinite Loop

What it is: A while loop whose condition never becomes false. Your program freezes.

# ❌ Infinite! count never changes
count = 0
while count < 10:
    print(count)
    # Forgot: count += 1

# ✅ Fixed
count = 0
while count < 10:
    print(count)
    count += 1          # This makes the loop eventually end

Fix: Make sure your loop condition will eventually become false. If stuck, press Ctrl+C to kill the program.

Modifying a List While Iterating

What it is: Removing items from a list while looping over it causes skipped items or crashes.

# ❌ Broken (Python) — skips items!
items = [1, 2, 3, 4, 5]
for item in items:
    if item % 2 == 0:
        items.remove(item)   # Don't do this!

# ✅ Fixed: Create a new filtered list
items = [x for x in items if x % 2 != 0]

# Or iterate over a copy
for item in items[:]:          # [:] makes a copy
    if item % 2 == 0:
        items.remove(item)

Assignment (=) vs Comparison (==)

What it is: Using = (assign) when you meant == (compare). In Python this is a SyntaxError in if statements. In JS/C# it can silently do the wrong thing.

// ❌ JavaScript: This ASSIGNS, doesn't compare!
if (x = 5) {       // Always true! Sets x to 5
    // ...
}

// ✅ Fixed
if (x === 5) {     // Compares x to 5
    // ...
}