I'm a professional music educator, and I see students make errors based on mistaken transcriptions all the time. Not just internet transcriptions, though: sometimes they transcribe something themselves and don't quite get it right. Often the problems are in the piano reductions put out by the publishers. I found my first mistaken time signature years ago in the sheet music for Sky's first album, and there is what seems to me a very questionable transcription of the rhythm of 'Recorda Me' in one of the Real Books.
As far as the internet goes, I seem to remember a while back looking at online tabs for 'Tie a Yellow Ribbon Round the Ole Oak Tree', and I couldn't find a single one where I could agree with the chords given.
I have a dream that one day someone will open source the entire history of recorded music – the origial stems, that is – so that we can go back and listen exactly what the keyboard player was playing in bar 7 that day. Oh, plus the original autograph scores to every movie ever made :)