Products introduced by MS that were massive commercial failures--neither becoming profitable nor achieving some strategic objective.  (MS didn't make money on IE--giving it away--but it certainly shouldn't be counted as a "flop").

* MicrosoftBob
* EuropeanMsDosFour
* MicrosoftChromeFX - Never became a 3d standard, although it arguably achieved the strategic objective of preventing everyone else.
* MicrosoftTest - If you've ever used it, you know why it's a flop
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They named one of their fonts "Trebuchet MS". No wonder Windows crashes.
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See Also: MicrosoftKudos