On 'Cotton Crown' by Sonic Youth, Thurston Moore and Kim Gordon sing 'You got your cotton crown' multiple times at the end. Gordon accidentally sings it one too many times and trails off, like, 'You got your cott — uh'.

In 'Maxwell's Silver Hammer', on the second verse Paul McCartney laughs while singing the word 'writing', reportedly because John Lennon mooned him from the control booth. Another Beatles one is on 'What Goes On' when Ringo Starr sings 'Tell me why' and you can hear Lennon shout 'We already told you why!', presumably in reference to their earlier song 'Tell Me Why', which Starr also sang.

There's a really famous one on Nirvana's cover of 'The Man Who Sold the World'. At the beginning of the guitar solo, Kurt Cobain misses the note then overcorrects and misses it again, but it's a surprisingly musical-sounding error. I doubt he'd have dubbed it regardless!