Program where I had three massive text files with data all mixed up in them. I had to insert enough markup to identify the parts of the data so I could load them into a database.
The text files were forum topics of questions and answers I had downloaded from a forum. The idea was to separate the questions from the answers (i.e. have a question database and then an answer database) and thus combine the questions and answers programmatically in response to a user search. The problem we had was that searching the forum topics themselves would return these massive posts with seemingly unrelated questions, thus if you wanted to find anything topically it took forever.
I'm getting a headache just thinking about it. What I learned is that there are more ways to format questions and answers in an forum post then there atoms in the universe, more ways for a markup insertion program to fail than any other one, and basically...yeah, that takes the cake. I got the markup inserted and then basically gave up - the markup insertion program was a complex feat of itself. Maybe someday I'll get back to the database entry. Fortunately, I wasn't getting paid for that job, so I could bail out.