"I was just reading about Willy Widdershins's arrest when you arrived. You know Willy turned out to be behind those regurgitating toilets back in the summer? One of his jinxes backfired, the toilet exploded and they found him lying unconscious in the wreckage covered from head to foot in -"
-- Arthur Weasley, changing the subject (OP22)
Muggle-baiting is activity which uses magic to confuse or humiliate Muggles without the Muggles realizing that magic was involved. Arthur Weasley describes it this way:
“Sell [Muggles] a key that keeps shrinking to nothing so they can never find it when they need it …. Of course, it’s very hard to convict anyone because no Muggle would admit their key keeps shrinking – they’ll insist they just keep losing it. Bless them, they’ll go to any lengths to ignore magic, even if it’s staring them in the face …. But the things our lot have taken to enchanting, you wouldn’t believe -” (CS3)
When Willy Widdershins rigged up regurgitating toilets, he and others were referred to as “Anti-Muggle pranksters” by Arthur:
“Anti-Muggle pranksters,” said Mr. Weasley, frowning. “We had two last week, one in Wimbledon, one in Elephant and Castle. Muggles are pulling the flush and instead of everything disappearing – well, you can imagine. The poor things keep calling in those – pumbles, I think they’re called – you know, the ones who mend pipes and things” (O7).