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1.
when something unexpectedly good comes from what would otherwise be considered a mishap.
the discovery of guinness was a happy accident.
by Mike Atari August 08, 2005
 
2.
A pseudo-polite term for a person who was conceived by mistake. The term implies that this 'accident' was one of the best mistakes their parents ever made.
'Dave's just a tear in a condom mate, that's all he is.'

'I'd prefer to think of him as a happy accident.'
by Liamness October 14, 2009