OK, I've been reading this site for a while, and I don't have as many WTFs to share since I mainly deal with the hardware side of IT as opposed to the software side, but I think I finally found a genuine WTF worthy of this site.
Before the WTF, some background on its origin. I was servicing a POS system at a location of a convenience store chain with ~1700 stores that has just upgraded to this "latest and greatest" POS system, unifying at least four different systems that were in use and streamlining reporting and back-office functions. The software is produced by a company that ONLY makes this software. Publix supermarkets, Toys-R-Us, and Tesco's use this same software, just different variations of it, so this isn't a student or otherwise project.
The problem I was called out to fix was a PIN pad not communicating with the host POS. 9 times out of 10, someone has unplugged the USB cable from the POS. That results in a normal error message like "PIN ENTRY DEVICE NOT PRESENT". Occasionally, a PIN pad will fail, and the terminal will get pretty creative with error messages like "UNEXPECTED DATA FROM PED", or "PIN ENTRY DEVICE MALFUNCTION". These are usually accompanied by some nice diagnostic information that helps the technician understand what's wrong. Today, however, was different. When customers attempted to enter their pin, it would look like it was processing the card, but the terminal would give this error: http://i196.photobucket.com/albums/aa6/theikitsune/IMG_0052.jpg
That was the entire error message. No debugging information, no nothing. And it didn't even mean what it said on the tin; selecting "CLOSE", and then retrying the transaction worked flawlessly.