One of my favourite tropes in comedy is scenes where characters are talking at cross purposes without realising. My favourite example of this is on This Country, when Kurtan doesn't want Kerry to know he has been talking to the vicar about going to college, so he tells her they were talking about the vicar's ball bag instead:
The only other one I can think of is when on Friends, Ross tells Rachel he has had lots of dirty dreams about him and Charlie, but she thinks he's talking about her and Joey.
I know there are tons of other funny scenes like this but can't think of any. What are your favourites?