Did a number 12 seed ever beat a number 5 seed in the ncaa tournament?

Yes.

In fact,

in the 2013 NCAA men's Basketball tournament, the #12 seed beat the #5 seed in three of the four regionals:

California over UNLV 64-61 in the East; Oregon over Oklahoma State 68-55 in the Midwest; and Mississippi over Wisconsin 57-46 in the West.