3
Wait, wait, wait, I think I figured out where Letty might've gotten that.
In the nakama vs. nakama arc (a.k.a. where Laxus and his three cronies are making everyone in FT fight one another), Fried puts up a boundary thing, and anyone 80 or older (Makarov) or too weak (I think it was this?) could not enter. And because Laxus thinks Natsu and Gajeel too weak (that, or else very powerful, thus having Fried create a boundary that doesn't allow those who are too powerful in), they can't get in. Now, Erza might count as too powerful, but since her magic eye's what got her out of being in stone, it would make sense that she could get through without a problem.
/why is that like, the only thing I can remember about that arc?