Yesterday on MTGO an ability didn't trigger and I don't understand why.
I had Experiment One in play, with one +1/+1 counter on it, so it was 2/2. Experiment One has Evolve (Whenever a creature enters the battlefield under your control, if that creature has greater power or toughness than this creature, put a +1/+1 counter on this creature).
I also had Strangleroot Geist, without counters. It has Undying (When this creature dies, if it had no +1/+1 counters on it, return it to the battlefield under its owner's control with a +1/+1 counter on it.)
Strangleroot Geist then died. So it goes. Its ability triggered, and it returned to the battlefield with a +1/+1 counter on it, making it 3/2.
At this point I expected Experiment One's ability to trigger, as it is 2/2 and a 3/2 enters the battlefield, but it didn't.