They can assign you a reserve trip into the next month (when you transition to a lineholder) and take away a day off. So while technically not a HDO (lineholders have days off but they aren't specifically called HDO), I will argue that it's essentially a HDO. Had this happen several times and had to cancel plans on those days off.

They can also take away next month's line awarded trips so they can work you harder as a reserve in the current month.