A couple of things to add:

A phrase of mine referring to the method described by Craig: " I do not code a project, I grow it"

Keep ISRs short (and sweet)

Any unused interrupt can be used as a low priority "software interrupt" which can take "the long" out of a hardware interrupt.

Have a routine void Crash(void); set a breakpoint there and whenever it is called find the callers address in the stack.
Uses of Crash():
At default:
If a function will only work if a variable is less than 10 (e.g. array offset), then insert: if (var > 9) Crash();

Play it safe, have fun,

Erik