I have done this twice, once when I was in F on a uni trip to Shanghai and one of my classmates was in Y - we had a lot to drink and he stayed a while - was a lot of fun.

Then last year I was in Prem Economy to Lagos and some people who work for me were sat in F (don't ask!). I needed to sit and work with them and was allowed through for a while, although a somewhat sniffy CSD told me to be very quiet as some people were sleeping.