The nesting DO (or function call) levels is limited to 128 (if I remember correctly) in VFP. You'll get this error (usually) when a function is called recursively or you have a ring of function calls.

Practically, it's very easy to get this error when you have methods that call each other or A calls B calls C calls... calls A. The easiest way to find it is to step into the code using the debugger. Set a breakpoint at the begining of each function involved. Even if the call ring is large, it's impossible to not find it in this way.

Vlad