What are some mistakes that were left in songs? Do you have a favorite?

You can hear a telephone ring in the background of The Ocean by Led Zeppelin.
- I always wondered what was up with that

The lead in to Good Riddance by Green Day is messing up the opening twice in a row. He even swears after the second fuck up.
- I always thought it was a deliberate part of the track, but the internet insists it was a genuine double mistake and f-bomb from Billie Joe. Nice!
- Every time I play this song live, I do it with the "...fuck"

Just a tad before 3 minutes into 'Hey Jude', you can hear someone deep in the mix say, "Oh, bloody hell".
- @darthlink @favrion
  That was Paul. He hit a clunker on the piano and said "Fucking Hell!". John insisted they leave it in. Google F word in Hey Jude.
- I never heard that one before! Is it two people? Sounds to me like Paul saying 'Oh!' then John mumbling 'bloody hell.'

In the beginning of Roxanne from the Police, Sting accidentally sat on a piano, and you hear the butt chord and laughter.

Mines a funny one because it probably doesn’t exist.
When young I was in a band that did a cover of Fire by Hendrix. And while practicing it along with the recording I would sware that there were moments where Mitch Mitchell went out of time for a beat or so. I never nailed down where it would happen because I was just trying to learn the song, but it felt pretty real though transient. It was probably me, but it would make total sense given the time and the nature of the song. Tempo changes are pretty common for instance for that time.

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!

In radiohead's Creep when the first verse shifts to the first chorus, the guitar is supposed to start playing during the chorus. But the guitarist apparently didn't like how quiet the first verse was so he played the first 3 notes a little bit early to surprise everybody. Some also say he was probably just checking the volume on the amp. It's not completely clear what his intentions were. But they left it in the final recording and even played it like that live.

On Dream on (Aerosmith), I've always wondered if the couple "missing" notes from the arpeggio in 2:33 are intentional or a mistake.
- I think that may be intentional for crunchy harmony.

At the end of Streetlight Manifesto's "Keasbey nights" when one of them says "it fucking stinks in here!"