showstopper

Noun	1.	showstopper - an act so striking or impressive that the show must be delayed until the audience quiets down Synonyms: show-stopper, stopper
	2.	showstopper - something that is strikingly attractive or has great popular appeal; "she has a show-stopper of a smile"; "the brilliant orange flowers against the green foliage were a showstopper" Synonyms: show-stopper

showstopper - A hardware or (especially) software bug that makes an implementation effectively unusable; one that absolutely has to be fixed before development can go on. Opposite in connotation from its original theatrical use, which refers to something stunningly *good*.