I don't seem to have ever linked to the brilliant O Fortuna misheard lyrics video, which plays "O Fortuna" while showing what the lyrics might be. Plus visual interpretations of those lyrics. Not knowing the actual Latin lyrics, it's very easy to be pretty sure that the lyrics they're showing are in fact what's being sung. Remarkable how suggestible I can be if shown printed words.
Anyway, what brought that to mind was that Alastair linked to a video of The Marmoset Song, a.k.a. "Marmoset there'd be days like this." It's only one phrase that they're mishearing/changing, but it makes for a great running joke, especially combined with the marmoset photos. Well done and very entertaining.
It appears that there's a whole genre of misheard-songs videos. For example, here's "Take on Me"—but the audio track has been removed as a copyright violation, so you'll have to play your own copy of the music while watching the video. It's a little hard to get them to sync up; I recommend waiting 'til the first frame of the video that shows words, then clicking pause on the video and waiting for the music to catch up. Some of the "mishearings" in this one are pretty far-fetched (that is, I can't imagine actually mishearing some of the words in that particular way), but some others are very funny.
Sadly, some of the other videos in this genre seem to be about making up lyrics that sound vaguely like the originals. For this joke to work best, the lyrics shown on the screen should plausibly sound like the ones being sung. Imo, anyway.