1) Your 8 month old will put a leaf in his mouth, start choking, sort of briefly stop breathing, choke some more, you will freak out, do the baby heimlich (full on pounding his back while he's flipped upside down), freak out some more, swipe your finger back in his throat further than you thought it could go until you finally get the blasted thing out.
2) Your 3 year old will get suspiciously quiet and upon investigation he will have the bottle of baby Tylenol that you meant to put away but hadn't yet in his hands with the evidence on his face, with a third of the bottle gone.
3) Your 3 year old will get into the knives that you thought were out of his reach. They now sit more than three feet above where he could ever climb...
4) Your 3 year old will pick up your 8 month old and place him on the couch.
5) Your 3 year old will put a grape in your 8 month old's mouth.
6) Your 3 year old will put playdough on your 8 month old's high chair, who then eats it, and when you discover this you see it's all over the roof of his mouth and playdough + saliva = trickiness.
7) Your 3 year old will kick a ball and it will hit your 8 month old (who is five inches away) directly in the face.