Hi Peter,
The way I see it as a newcomer, Haskell shifts the typical imperical programming bugs like null pointers and buffer overruns towards "space/time leaks", causing programs that either take exponentially long to complete, stack overflow, or fill up the swap file on disc because they consume gigabytes of memory.
Time bugs are quite rare - usually a simple profiling will fix them up, and they are exactly the same sorts of bugs that exist in an imperative programming language. Usually its a case of picking a better algorithm, or thinking clever thoughts. Space leaks are much more tricky - there are profiling tools, but I've never got enough experience using them to say anything more than that.
Other bugs I found are incomplete pattern matches at runtime, but I already got an email of how to fix this using an external tool
Did the email suggest using Catch? http://www-users.cs.york.ac.uk/~ndm/catch/ If you care enough about pattern matching, you can eliminate them all statically. Thanks Neil