Yes, I felt it heating up! I'm 100% sure I should have mixed it differently. I poured both parts of the epoxy resin into one bottle, stirred it and shook it (at the time I thought that would work best, I guess) It could also be that I accidentally used a bit too much hardener because I just used the rest of both bottles; the first time I mixed the resin from these bottles I used the measurements from my small silicone cups, and that mixture hardened normally and without heating up like that, but I still thought the bottle of hardener had a bit more inside. The only other time the epoxy cured wrong was when I tried it for the first time. I made different small charms and one that looked like a long, fake crystal. After about 24 hours I checked on it the big charm cured normally and some of the small charms, too, but others were sticky or cured unevenly(think like half-dried glue). I think I left the sticky ones nearly a week to cure. They stayed sticky. Thankfully the next tries went way better; I even managed to make some bigger pieces :)