What if somebody used counterfeit/proxy basic lands in an official MTG tournament?
The tournament rules only allow official WotC product and make no distinction between basic lands and others. What would actually happen in a sanctioned event is ultimately the decision of the head judge, as there is no explicit penalty for using counterfeit cards that are indistinguishable from real cards.
From the MtG tournament rules, section 3.3 &quot;Authorized Cards&quot; and 3.4 &quot;Proxy Cards&quot;, it is clear that only official, genuine cards are allowed in a tournament. A player can alter the card to a degree, as long as the card remains unmarked and essential information remains legible. When a card becomes marked/illegible during a tournament, the head judge may issue a proxy card.
However, the Infraction Procedure Guide only lists the penalty for using marked cards, the consequences for which range all the way from a simple correction to game loss, depending on the circumstances. There is no explicit penalty for using counterfeit cards that would be indistinguishable from the real cards, i.e. when they could not be considered marked.
Since using counterfeit cards is not allowed by the tournament rules, but the IPG does not cover that specific case, the head judge would have to decide on the penalty, depending on the circumstances, so a general answer cannot be given based on the official rules alone.