Reanimating a creature with X in its cost
It would enter with 0 +1/+1 counters, then get put back into the graveyard for having 0 toughness as a state-based action.


  107.3f If a card in any zone other than the stack has an {X} in its mana cost, the value of {X} is treated as 0, even if the value of X is defined somewhere within its text.