Hardware backward compatibility: The firmware that missed one tiny detail
The person responsible for the floppy disk driver in Windows 95 also was responsible for the low-level CD-ROM drivers. (Not to be confused with the CDFS file system, which was handled by the file system team, not the hardware driver folks.) And I remember a story about one particularly strange CD-ROM drive. This drive was produced by a name-brand manufacturer. The box that the drive comes in proudly announces that it is an IDE ATAPI drive. And they did a fantastic job. They implemented all the ATAPI commands that were defined at the time, with one tiny exception.
They forgot to implement the “Are you an ATAPI drive?” command.