An example that bites Haskell programmers are space leaks due to too much partial evaluation and not enough full evaluation. Other problems include keeping a file open for too long and leaking the fd. An ideal computer has infinite space and no files. A real one... nope :)

The advantage of functional programming languages is that you get to write part of your program as though you have an ideal lambda calculus evaluator. But at some point, you still have to realize that there's a grey box under your desk that doesn't know what a lambda or fixed point is. (But this is better than programming languages that say "hey, you've got a heap, a stack, and some registers! well, so long!")