There clearly are a lot of examples where beautifying has been done for reasons of preference, but I think this can also verge into people just being unfamiliar with canon or seeing things no one else is seeing. For example, if I didn't know what James Marsters looked like I would probably rarely guess through reading fanfic alone. Two descriptions I remember in particular are of his long legs and elegant hands.

In SPN, where you have two incontrovertibly tall and good looking guys I've seen a few odd things said as well. The one I've seen most often is how Sam is dark-skinned. Granted he probably tans whereas Dean burns but he is anything but dark skinned. In one fic he was in disguise and his skin described as cinammon toned and I did not figure out it was him until he was revealed because I couldn't square that description with the character.