Another option would be to pull the file in question up in ghci and play directly with the components of the function - in pure code that should be safe in a way that it won't be in effect-full code.

I also dispute the assertion that QuickCheck doesn't help - if you're getting weird behavior, then hopefully you can characterize that weird behavior, and get some example failed values out of QuickCheck - and getting a picture of what values the function is failing for can absolutely be useful.