I was flying HEL-LHR once in the front of the cabin, and someone put their bag overhead in the business section before walking away and has been reminded by someone already sat in their seat that this was poor etiquette.

This was the first of April, I was tempted to ask the FA to play some April fools by putting said (small, under-the-seat) bag in the suit storage cabinet at the front. At that point I already had a couple of G&Ts in the Finnair lounge so I did zip it, as my good judgment was in question. But now that I think about it, doing so would've been... interesting.