"False etymologies" are quite fun, and there's a lot of them around that are "common knowledge".

Here is a site with a quiz about that kind of stuff.

My favourite one is the "curmudgeon" one chronicled in Brewers. Dr Johnson in his dictionary wrote, quite correctly, that it was derived from the French expression "coeur mechant" and credited it to "unknown correspondent".

A contemporary Dr Ash rushed out a rival dictionary, ripping off Dr Johnson's left right and centre - and he gave the game away when it came to his definition of curmudgeon - "coeur" - unknown, "mechant" correspondent, so that the whole word was taken to mean "unknown correspondent.

Talking of curmudgeons, which we weren't, but I've got round to it now, here's a site which I just found which could be interesting for some Mudcatters, I think: The Folk Curmudgeon Review