GCC/G++ compiler: Error goes away when run through debugger

I have an C++ application program that is producing incorrect results
but the error goes away when I run it through the debugger.


I'm using the gcc/g++ compiler under Cygwin (an UNIX emulator that
runs on windows).


When I used the -ggdb compiler flag so I can run it under the gdb
debugger, the error goes away. It also runs fine if I do both of the
following; extract a few of the input instances which generate
incorrect results and put them in a separate input file.
Use this input file and recompile the program with no optimization
flags (I am otherwise using the -O3 optimization flag).


The -O3 optimizations are supposed to always produce the same
results. Running through the debugger is not supposed to change the
results either. Has anybody else encountered this behavior?
[The usual problem is that you're doing something that isn't valid
C, so the optimizer doesn't preserve its semantics. -John]

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