Experienced programmers aren't immune to these; they just recognise them faster.

Bug	Symptom	Fix
Off by one	a loop runs once too many or too few times	check `range` ends and `<` against `<=`
`return` inside the loop	a function decides after the first item	move the final return after the loop
Accumulator reset in the loop	totals and counts are too small	set it up before the loop
Text from `input` not converted	`"5" + "3"` gives `"53"`	`int()` or `float()` first
`=` instead of `==`	SyntaxError in an `if`	two equals to compare
Changing a list while looping over it	items get skipped	build a new list instead
Naming a variable `list`, `sum` or `max`	"object is not callable" later on	choose another name
Forgetting to `return`	the result is `None`	return the value

Changing a list while looping over it is the sneakiest. Watch what happens:

numbers = [1, 2, 2, 3]
for n in numbers:
    if n == 2:
        numbers.remove(n)
print(numbers)

One 2 survives, because removing an item shifts the rest left and the loop skips the next one. Build a new list instead: numbers = [n for n in numbers if n != 2].