At the moment for me it's Teenage kicks. I always jump straight into a chorus instead of the verse later in the song, much to the amusement of my band mates. We're also a 4 piece with only one guitar, so it's REALLY obviously when I screw up.
when I was younger it used to be Wish you were here by Incubus. It wasn't even a difficult bassline!
Why is it always the simpler songs that are easiest to mess up?
I think my brain just switches off...