When resolving the Enters the Battlefield ability of Gyruda, Doom of Depths, if the only legal target is Progenitus, the game locks and you are unable to resolve the ability. Presumably this is because Gyruda's ability is not worded as "may," and so it will not allow you to proceed without selecting a target if the game believes there to be a valid target. However, when attempting to select said target, if Progenitus is the only card milled with an even mana value, it is not available to be chosen, presumably due to it's ability shuffling it back into it's owner's library. The game becomes stuck in the sceen to select a target, despite no targets being available to select, and will not allow you to proceed.