There is a dark secret with the song "Girl" according to Beatles Anthology

In the song, you'll hear the band repeatedly saying "tit" as a vocal harmony. That apparently was done on purpose according to the book Beatles Anthology and nobody ever complained or noticed.

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During an interview, Paul stated

"It was always amusing to see if we could get a naughty word on the record: ‘fish and finger pie’, ‘prick teaser’, ‘tit tit tit tit’. The Beach Boys had a song out where they’d done ‘la la la la’ and we loved the innocence of that and wanted to copy it, but not use the same phrase. So we were looking around for another phrase, so it was ‘dit dit dit dit’, which we decided to change in our waggishness to ‘tit tit tit tit’, which is virtually indistinguishable from ‘dit dit dit dit’. And it gave us a laugh.

It was to get some light relief in the middle of this real big career that we were forging. If we could put in something that was a little bit subversive then we would. George Martin might say, ‘Was that “dit dit” or “tit tit” you were singing?’ ‘Oh, “dit dit”, George, but it does sound a bit like that, doesn’t it?’ Then we’d get in the car and break down laughing."