you get different songs stuck in your head all throughout the day without even realizing it unless you stop and ask your brain what song you are singing at that moment. some of them meld together because of similarities that your auto-pilot discovers and subtly mixes without you even knowing and pretty soon you have learned the song your new wrong right way. your brain keeps singing it in the background and by the time you are conscious of it you can no longer tell the difference between your version and the original.
for example: this is the reason that you can't understand why your anal friends are annoyed when you scream "OHIO!" at the end of the opening credits for That 70's Show.