Drawing a card when a player is already at the hand limit.
If a player draws a card when they already have 10 cards in hand, they will overdraw. In normal game circumstances, the overdrawn cards are discarded into the graveyard; however, they do not proc the discard mechanic.
If cards are being drawn at the beginning of a new round and would put the player over the hand limit, the draws are instead converted into extra mulligans.
Cards that might force overdraw are Isbel of Hagge and Stregobor.