What happens if a creature with a +1/+1 counter and a -1/-1 counter simultaneously has both removed by state-based actions?

State-based actions include a rule that causes +1/+1 counters and -1/-1 counters on the same permanent to annihilate each other. Specifically, CR 704.5q states: if a permanent has both a +1/+1 counter and a -1/-1 counter on it, N of each kind are removed, where N is the smaller of the two amounts. This happens as a state-based action check, before any player receives priority.

So if a creature has exactly one +1/+1 counter and one -1/-1 counter, state-based actions remove both simultaneously — one of each — leaving zero of either counter on the creature. The creature's base power and toughness are unaffected; only the counters disappear. The creature itself remains in play (assuming it has positive toughness after the removal).

This removal is not a triggered ability and does not use the stack — it simply happens as part of the state-based action check. No player can respond between the moment the game checks state-based actions and the removal of those counters.

Concrete example: You control a 2/2 Grizzly Bears that has received one +1/+1 counter (making it effectively 3/3) and one -1/-1 counter (making it effectively 2/2 again). When state-based actions are checked, CR 704.5q removes both counters simultaneously. The bear is left as a plain 2/2 with no counters — not a 3/3, not a 1/1, just its base 2/2.

If instead the creature had two +1/+1 counters and one -1/-1 counter, only one of each is removed (N = 1, the smaller amount), leaving the creature with one +1/+1 counter and zero -1/-1 counters, per CR 704.5q.