This one is pretty well known. During 'The Lion King,' there's a scene where Simba crawls up a mountain to take some well-deserved rest. As he does so, some dust blows up into the air, and briefly appears to form the word 'sex.'
When this was pointed out to Disney, they tried to insist that it actually says 'SFX,' and it's a note of thanks to their animation team. That doesn't seem likely to us; however, you can take a look at this picture and judge for yourself.