There is one problem with this idea, in fact, a MAJOR problem.
Lets say, in some case you actually did make lots of lag which made repitition possible. This is the problem:
-end quest
wipe quest from board
give xp and gold
-end quest
wipe quest from board
give xp and gold
-end quest
wipe quest from board
give xp and gold
Notice how there are multiple end quests. Now see if you "end quest", you can't end it over and over, once the tag is activated it cannot be activated by the same character again. Therefore, the server would catch up to itself and correct the error.
At least you were thinking though