Using the word 'I' when it should be 'me'.
All these sentences are of a type I read often, and they're all incorrect:
Joe came to visit Bob and I last week.
The impression given to Bob and I was that Joe had stolen it.
Joe didn't tell Bob and I that he'd left
Joe took Bob and I to the pub
Things started to go wrong for Bob and I after the war
I think people make this mistake because, if their mothers were like mine, they were always getting told off for saying "Me and Bob are playing in the garden." Mother would say, "No, Bob and I are playing in the garden" - which is as it should be, of course. But it's not always right. Some people use it simply because they think it sounds 'posher', having the impression that "and I" is always correct.
It's easy ~ If you're not sure if you should write 'Bob and I' or 'Bob and me', simply take out the 'Bob and'. See what I mean? As soon as you read 'Joe took I to the pub' you realise that it's wrong. Thus, it should be Bob and me in all five examples listed above.
I see this mistake even in professionally edited books; I even saw one in a Phillipa Gregory! Yes, yes, I daresay her editor might find the odd error in stuff I've written, too, but it won't be this one.
Joe came to visit Bob and me last week.
The impression given to Bob and me was that Joe had stolen it.
Joe didn't tell Bob and me that he'd left
Joe took Bob and me to the pub
Things started to go wrong for Bob and me after the war