A bug in a computer program in development which is so horrific, so embarrassing, that the developers are literally unable to release the product until it is fixed. Showstopper bugs generally include horrific crashes in common tasks, messy things that would alarm people to the point they would be less likely to buy the product, and any kind of data corruption.

Documents from the development of large software projects often contain statements like "There are still forty outstanding serious bugs, three of which are showstoppers."

I once saw an account of a tester for a game company who had been handed a horrifically buggy game to test, and was told it was going to be shipped in three days no matter what. The tester, upon playing it, realized it was so horribly buggy that he had some kind of moral obligation to prevent it being put in the hands of consumers. He then spent the next two days feverishly trying to find some way to effectively mercy-kill the product by locating a showstopper bug so that he could force them to delay the product and run another bug fixing cycle. He eventually triumpantly discovered that running ctrl-alt-del at any point while the game was running would shut off the computer, and presented it to the dev team. They shipped it anyway.

"Show-stopper bug" is a common form of the same term.