I once wrote a Perl subroutine that would memoize the subroutine that called it. That much was useful, but then it inserted a copy of itself into the caller, so that its callers would be memoized too. A well-placed call to aggressively_memoize could back-propagate to the whole codebase, spreading functional purity like a virus. The resulting program would get faster as it used more memory and became increasingly static.