TAILENDERS! The bane of DXpedition operators.
The number of times I have almost all of a caller's callsign and the Tailender repeatedly jumps on him and makes it extremely difficult to complete the QSO. It is usually useless to address the offender (in CW) as he often does not understand anyway. My strategy is to refuse to acknowledge the Tailender, continue to call the station and make sure I complete with him, and then change my listening frequency. I'd rather my rate falls through the floor than reward the rude behaviour of the Tailender. They are usually extremely persistent too. He'll track me down and play his game again. Again I will not reward him. After a while, when I might find the tailender in the clear, I will log him.
Having been many times at the "Sharp End", it gives one a better idea and understanding what may be happening when one is the chaser. Most of us don't have an appreciation of what the DXpedition operator hears at his end of the pileup.
Most of these rude and inappropriate behaviours manifest at the start of DXpeditions. It usually settles down a week in.