This happened yesterday on my ORD-YUL flight. But it happened only after all of the J class passengers had been seated. There was (surprisingly) still room in the overhead bin, so it didn't really bother me that someone put their bag in J class. They were clearly out of room in coach.
This was my first time on the E175. I didn't realize just how small the overhead bins are in J class on the port side of the aircraft. For this reason, I checked my bag on the return since I just didn't feel like worrying about space filling up. It turns out there J class was only half full, so checking my bag was not necessary. The good news is that it took all of three minutes for my bag to show up on the carousel once I got there.