About one in 55 billion, in fact.reanjr said:18 on 4d6, drop lowest = 21 in 1296
21 in 1296 ^ 6 = 85766121 / 4738381338321616896
Not quite never. Better than one in a trillion.
I'm not sure how Halivar came up with 7.15 in one million, but that one was way off.
If this were ever to happen to me, I would apologize to the DM for using a set of dice which were obviously biased, discard them, and roll again with a different set of dice.