When I was working at the help desk at the computer center as an undergrad, the professor in charge of that group used to give us interesting little language tests for things we needed to watch out for, especially with beginning programmers.
One of his favorite ploys was to use the EQUIVALENCE function in FORTRAN to equivalence a constant with a variable, then assign something to that variable. In one of the FORTRAN compilers, that would result in overwriting the constant, so all future uses of it would have the new value. This would break many things, of course.