Thread: corrupted stack???

Hi,
I'm having a little problem. When my program is terminating (in debug mode) MSVC++ 2008 gives me the following message: Run-Time Check Failure #2 - Stack around the variable 'originalExtension' was corrupted.
originalExtension is an array with 21 elements that holds a string (20 characters + null). I read a string from a file using the following line:
fgets(extension, 21, readfile);

What does this error mean? my program seems to run normally, and the error appears only at the return statement in main().