We've got an app that imports data from CSV. I didn't write the import - presumably they could have made it support Excel as well. That's not the WTF.
One of our users has been complaining for a while that she was getting weird behavior when she imported. We investigated (using our own CSV files) and figured out that we'd been bitten by the old issue where Microsoft Jet tries to guess the column types, and then discards any data that doesn't match the guessed types. That's *a* WTF, but not *the* WTF.
In order to make this change on the app we had to change a registry setting on the server. It was completed by IT yesterday and she continued testing. However, she complained that the application was still importing junk. We verified the settings, imported another file, etc. Finally one of the engineers asks her to send him the file she's trying to import, so we can see if there's something unusual with it.
When the file arrives, he opens it in Notepad to inspect it - it's only 27K, and she claims it should have five columns and about a hundred rows. When the file opens, we see nothing but random junk. It's obviously a binary file. So I had him rename the file from "import.csv" to "import.xls". Hey, guess what - Excel likes the file just fine.
So following some swearing and laughter, we informed her that she can't just change a file extension to "CSV" if she wants a CSV file.