Below is a photo of poison ivy I have growing at my home. Notice anything strange about it (other than I am purposefully growing poison ivy)?
The second photo shows it best: there is a group of only 2 leaflets instead of 3. Poison ivy always has 3 leaflets, but sometimes one can fall off, get bumped off, or eaten (humans are the only ones who react badly to this stuff; in fact, deer and rabbits supposedly love it). Point being that poison ivy always has 3 leaflets unless something has damaged the plant.
I don't know what happened to this leaf on my plant. God knows I didn't touch it!