Common mistakes are an indication of poor language design

English mistakes:
They're, their, there
You're, your
It's, its
till, til

Programming mistakes:
Null pointers and null pointer dereferencing
Uninitialized objects
Etc.

If something happens a lot, the program is the language itself, not the person using it. What common problems can you identify in the languages you use?

off by one
integer overflows
buffer overflows
null termination
out of bounds
escape characters
order of evaluation
implicit type conversion
dangling pointers
memory leaks
undefined behavior