In 'You're the One That I Want' out of 'Grease', I always thought they were singing 'Russian love, Russian love for me and you' (ie Nothing left, nothing left for me to do).
And going back a bit further than that, I thought the words to the nursery rhyme were 'Horsey horsey don't you stop. Just let your wheels go clippety clop.' Sad thing is, I was in my twenties and singing it to my niece before I suddenly thought 'hang on, horses don't have wheels do they?' Well, it's not like I sang it all that often in the intervening years..
A childhood belief about misheard lyrics

others believed things like this
- When Madonna sang "let the choir sing" in "Like a Prayer", I thought she was singing "love by proxy"— Kristen
- when asked what i would like to be when i grew up, as a girl of about 5, i answered "a horsey".Anon
- I thought all you had to do to speak Russian was add -ski to the end of every word!— nateski
- I used to believe that when people talked about 4x4 trucks, I thought it meant all the wheels turned.
— tracy - I used to think that Russian was just English backwards— Millie
- that the bears which live in the cracks between paving stones are Russian— jonah fonder