One of the ways that I do this (assuming you have access to unix utilities) is to do:

  cat output.log | tr -d '[0-9]' | sort | uniq -c | sort -n

This is a fairly useful way of removing relatively useless information such as timestamps and line numbers when you're looking for rare or unique events. The alternative, I think, is to do a bunch of awk or sed magic, which isn't really fun for anybody. It's especially useful in a time crunch when there's an ongoing outage.