If an opponent controls Zur's Weirding while you have a card with dredge in your graveyard, it becomes impossible to carry on with the replacement effect of Zur's Weirding. When clicking Zur's Weirding to select it as the replacement effect, it drops the name of the revealed card in the log and returns you to the option of which replacement effect you would like. This process repeats until the active player chooses to dredge.