There is another mechanism in Keil (and probably other 8051 compilers) that can crash your app long before the stack overflows!

It is because Keil C51 does not use the processor's hardware stack for parameters or local variables - it uses fixed memory locations instead.
Obviously, this makes everything inherently non-reentrant!

This also makes some big issues for any use of function pointers in C51; see http://www.8052.com/forum/read.phtml?id=47543 for some very important references that you must read before using function pointers in C51.
As Peter Dannegger subsequently poitns out, it is perfectly possible to do it in C51 - but you need to understand the issues if you're to avoid the pitfalls and some really weird bugs!