Senior FO's watch for TOE trips. The day prior to the trip they call the junior FO on a TOE trip and swap their better trip with him. At 7 pm that night they are now the senior FO and stay home. The original senior FO now flies a trip he thought he would be home for. There are a few guys who do this all month long. One FO I know lost staying home on two trips in a row to the same guy.
It's legal but I would not call it ethical. Easy fix would be the senior original line holder on the trip stays home unless both guys dropped it.
There are some very ****ed off FO's over this issue!