I ran into an interesting bug with MS excel the other day. Turns out several versions of excel have issues with CSV files which have ID in capital letters as the first two characters of the file. Excel throws an error, "SYLK: File format is not valid".
One of my coworkers ran into the error when they tried to open a report I had sent them. It took a bit of time to track down, and made me question my ability to generate CSV files for a little while, but at least it was an easy fix. I just changed the first two characters "ID" to "id".
The two things I thought was really entertaining about this bug are first, you can export an excel file which had "ID" in the first cell to create an invalid SYLK file and second SYLK files are a Microsoft file format.
If you are interested in some more details here are a couple reference links:
Microsoft Support page
Wikipedia article on SYLK files