Food mystery

My cats Kaci and Mittens don't normally eat Fancy Feast for breakfast but this morning I gave Kaci some FF Oceanfish & Tuna because she seems tired of her usual breakfast food. She had some FF Oceanfish & Tuna yesterday evening and had no problem with it. I wasn't paying a lot of attention to her but she either didn't eat the food or only licked it or smelled it. It didn't look touched. It was her reaction to it that was strange. She walked away and began gagging and drooling and threw up foam. Afterwards she seemed perfectly OK and was able to eat something else and has appeared fine since then. A little while later Mittens approached the food and sniffed at it while I was still cleaning up from Kaci. Mittens doesn't like FF Oceanfish & Tuna so it didn't surprise me that she didn't eat any. But she had the same reaction as Kaci. She began gagging and threw up white foam. Again, Mittens was ok afterwards and has no signs of any illness. I looked at the food and it didn't look strange. It smelled stinky but to me didn't smell any stinkier than it usually does. What could possibly be in the cat food that would make Kaci & Mittens gag and spit up foam just by smelling it? Has anyone else ever had this happen with their cats?