The only real problem with Goblin saboteur is that you cannot always tell if your opponent raised his or her wall by more than eight. If you sign on to find that your opponent has played Reuse the rubble or Eternal guardian, there is no way (aside from having kept your own notes) to tell the amount of the increase or what the previous card had been.
Additionally, Castle beyond looking glass, Magic barrier, Order of the white lotus, and Return to the beginning leave no indication as to the amount that the wall was actually increased (or potentially decreased, though it would then be an orange number). Without having written down or otherwise recalling what the values were beforehand, it's anyone's guess whether the effect will occur.
(Another possible, though far less likely, scenario is for someone to play Basic Wall while having 7 wall, raising it to 10. Someone returning to a game from a previous day could mistake this for a wall raised from 0 to 10 and expect the conditional effect.)