Counting number of cards in a library

[Rune]

Am I allowed to count the number of cards in an opponent's library or can he refuse to let me do it and do it himself instead?

(assuming the opponent always has the right to do the counting himself, what happens when he miscounts? For example, my opponent says he has X cards left. I mill X cards, but he still has 1 or more cards left.)

[cdr]

You are allowed to count the number of cards in any zone. In fact, he can require you to count - it's derived information, and (except at regular REL) he isn't required to tell you.

If your opponent misstates derived information, it's a Player Communication Violation - he gets a warning. If it's possible to back up to the point of error without disruption, the game is backed up to the point of error.

In your example, I would feel ok with backing the game up.

[Meekrab]

Am I allowed to count the number of cards in an opponent's library or can he refuse to let me do it and do it himself instead?
Pile shuffle your opponent's deck before each game, then you know how many cards are in their library. And if they present anything other than 60 cards you should probably call a judge for a deck check.