As an example... I once wrote some MPI code for class. It ran properly at -O0, though the compiler warned of a variable that was declared but never used. Compi

As an example... I once wrote some MPI code for class. It ran properly at -O0, though the compiler warned of a variable that was declared but never used. Compiling at a level other than -O0 or removing that variable declaration from the source code caused the program to segfault immediately. It turned out to be a memory error somewhere else in the program (I forget exactly what, but it was over my misunderstanding of some part of the message passing calls).