"Can't Revenge" -- what gives?

[HeadOnFire]

So I see this "can't revenge" message popping up on my defense list from time to time. Usually it's just for individual players that I've fought several times, which makes sense; they don't want 2 players to just beat each other's defenses over and over for infinite coins. But once in a while my entire defense list gets blocked, even players I've never fought before. Is there a daily/weekly limit on your total defense battles, or is this just another bug?

[Sick]

You can only revenge 20 times a day.

[GotN]

Correct Sick. Sparton confirmed this here:

http://forum.ironmaidenlegacy.com/showthread.php?9262-The-Charge-Begins!-Trooper-Soul-Fragments-from-Arena!/page10