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?

Unchecked exceptions.