Ooooops

'A couple pf - what's this codswallop?'
'De-men-tors,' said Harry slowly and clearly. 'Two of them.'
'And what the ruddy hell are Dementors?'
'They guard the wizard prison, Azkaban,' said Aunt Petunia.
Two seconds of ringing silence followed these words before Aunt Petunia clapped her hand over her mouth as though she had let slip a disgusting swear word.

(OotP, UK edition, p. 33)