"no smoke without fire"
Apart from the fact that apparently the person who coined that phrase never forgot a bunch of cookies in the oven it's also incredibly unfair as it implies rumours are always true. They aren't! And forgetting that can cause so much harm!
A close friend of mine was libeled badly on a message board: he was supposed to meet with a girl he had met online and at the last minute she got cold feet and decided to take the very next flight home again. She didn't tell my friend, though. He had no idea why she didn't turn up, got worried and had her be paged at the airport. So they finally met after all, talked and left quite amicably.
Later, however, she spread the rumour that he had attacked her and she'd had to call the airport security. Some fools believed it and my friend got banned by the board administration. He registered under a different name to defend himself and immediately got banned again. After a few rounds of this he was so furious that he started trolling wherever he could. Not really surprising.
8 years after the incident the girl finally admitted to the libel but it was too late: my friend has a bad rep, suffers from persecution mania (he received death threats from board members!) and cathegorically refuses to post anywhere. A big loss as he was a rather interesting poster.
And all of this because a few people believed in this "no smoke without fire" nonsense. It makes me so furiuous!!