When I was little, I thought that the song "This Old Man" ended with, "Knick-knack, patty-whack, keep it on the phone" (instead of "give a dog a bone"). My little brother (two years younger) picked it up from me, and once I found out the truth, it took me the longest time to convince him of the real words. :)
A childhood belief about misheard lyrics

others believed things like this
- For the longest time I didn't know that Spinal Tap wasn't a real group. No wonder I couldn't find their music.— Louis
- i used to believe that a sign in a cafe bone-fide meal meant meat with a bone in it— roy
- Yep I thought the Zephyr song was fly away on my cell phone— Jenny
- I always thought that my relatives were really tiny, and inside my phone when I talked to them on the phone.Anon
- When I was younger, my cousin used to try to convince me that I was adopted and I sometimes believed it."Anon
- My brother that is 6 years younger than me used to believe that in seven years he would be older than me.Anon