Already mentioned so far:

Stack overflow;
Overflow of other variables.

Some more:

Uninitialised variables - especially pointers;

Missing 'default' in switch statements;

Mismatched PUSHes and POPs;

Problems with overlaid data;

Inadequate handling of "invalid" inputs;

Problems with pointers;

and many, many more.

There are just far too many things that might go wrong that it is impossible to say from the information you've provided.

You need to add some debug/diagnostic facilities to your code, so you can follow what it's doing and, hopefully, see where it's going wrong