System crash that occurs when you demo your (previously) bulletproof code for the boss, customer, or instructor (depending on environment).

Fallout from the LawOfDemos.

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One of the more famous DemoMeltdown-s was not so long ago when BillGates was demo'ing the PlugAndPlay USB features of Windows98 and he experienced the BlueScreenOfDeath when he went to plug the device in.

--DrewMarsh

As I recall, upon encountering the BSoD, he quipped: "That's why it's still in beta."  -- JeffreyHantin

The dialog from that most famous of demos is as follows. The co-announcer (real name anyone?) does the traditional "Woah...", after which they pause for the audience to settle down a bit (a lot of laughter), this is followed by the co-announcer's "Moving right along..." and Bill Gates saying "That must be, er... that must be why we're not shipping Windows 98 yet."  -- JohnKemp

See: CriticalNeedDetector