Worst are probably
(*) an upgrade that is gonna clear anyway, e.g. SFO-LAX on a sUA 757 for a 1K on an H fare on an empty weekday.
(*) an upgrade that is not gonna clear but will lock up your GPU for months, e.g. the time we applied a GPU on a W fare ORD-LHR while the cabin was booked 0/26 and gradually watched it fill up over the course of months while our upgrade never cleared