sniggerbolt
noun
Something that causes laughter not because it is funny, but because it comes as a surprise during a tense moment.
Sander was very nervous during his choral audition, so when the person who went next had a voice of a higher pitch than his appearance would suggest, it came as a sniggerbolt and Sander couldn’t help but laugh. The director shot him a dirty look and Sander knew he had no chance of making the cut.