Hi Robin,
When resin is smoking, it means it has heated up very quickly. (I’m glad you didn’t hurt yourself!) There are a couple of different reasons or ways this could have happened.
1. The manufacturer recommends mixing no more than 1 gallon total at once. If you mixed more than this, the mixture can heat up quickly.
2. Your resin/hardener could have reacted with something in your mixing cup. I have seen this happen with styrofoam and red party cups.
3. In your case though, something already likely happened with resin or hardener (whichever was cloudy). Neither should be cloudy, so it sounds some kind of chemical change already happened. Whatever it was likely caused the smoking you saw from your mixing cups.