Haskell's laziness can also be a notorious source of "space leaks": an algorithm that appears O(1) in space, such as summing up a list of numbers, can actually use O(n) space by accumulating unevaluated invocations of (+) in memory. With more complex data structures, the proportion of expected memory usage to actual memory usage can get even worse.

In larger Haskell programs, I've found that the most challenging issue to debug: "why does my program use way too much memory?".