[ryandrake]

As the sibling comment already mentioned, file format reading is a big one. Oh, a customer needs us to ingest CSV data. Let's just roll our own CSV parser, the file format is simple, right? What could go wrong? Same for graphics formats. We need to read PNG files, so let's just figure out the format and implement just enough to read the handful of files we have on hand. Even though libpng exists.

Another, more domain specific one: We have to talk to a GPS receiver that connects via RS-232 and outputs NMEA formatted data. This device, like all such devices, outputs a small subset of the standard. So parse just enough of it for that one device we have to get working, and ship it. Then someone attaches a different device that outputs a different subset of the standard and the software fails (sometimes gracefully, sometimes not).

[kruador]

That reminds me of the Windows Mobile 6.5 device with built-in GPS receiver, where on the 31st day of the month, the API would actually output the date as the 0th day of the following month. This wouldn't have been a problem, except that we were using that to set the system clock. The devices would reset their clock if allowed to go completely flat, but they didn't have reliable NTP support either. I believe the OS would just ignore our attempt to set the date to an invalid value, leaving it at the system default date.

It was a struggle to get the vendor to even acknowledge the problem, since it would only happen on 7 days out of the year, and usually 60 days apart. (There are only two consecutive months with 31 days, July and August.) It did eventually get fixed.

I seem to recall writing a workaround, but then it being stalled by our customer's Change Control Board.