One I found with profiling--when writing the code n was quite small. Many database operations simply iterated over an array to decide where to store an item. The time spent dealing with the data was a tiny fraction of the database round trip time, there simply was no reason to get fancy.

Over the years they've grown and one bin showed up where jobs would sit around for weeks--for that case n went from rarely having a screenful of lines to a few thousand--oops, now more than 2/3 of the time was spent in those searches. (And I have a sneaking suspicion that a good portion of the remaining time comes from using field names to retrieve values. The profiler doesn't separate that out, though, because it's not my code.)