When I worked in a daycare a long time ago, we had a little one in the infants room that when he got upset, he would literally stop breathing, his lips would even turn blue, we would have to blue puffs of air into his mouth as soon as this would start, to encourage him to take breath. It is very scary and thankfully not all that common. He did grow out of it eventually, but it was nerve racking everytime he would get really upset.