What's a bug you shipped that was technically correct and completely wrong?

Mine, from this week.

I filter email security scanners out of my analytics — Safe Links, Mimecast and friends open every URL in an email before a human ever sees it. So I matched "outlook" in the user agent. Obviously a scanner.

Except Outlook's mobile apps put Outlook-iOS/745.0 in the UA of a real person reading their own mail on their phone. Which is where most B2B mail gets read.

So the check was correct about the string and wrong about the world. It took the most engaged reader I had and reported "never opened."

The lesson I keep relearning: the string that identifies a bot usually also identifies a person using that same vendor's software.

What's yours? Looking for the ones that passed every test and were still wrong.