Carpet damages small mobs, including wolves, ocelots, chickens, spiders, pigs, and silverfish. Large mobs and humanoid mobs are not affected. I had tamed wolves sitting on carpet, and after updating to 0.12.1 the carpet killed them. Occurs in creative as well as survival.
Steps to reproduce:
Place a few carpet tiles (tested with white and red), then spawn one of the affected mobs over the carpet. The mob will take damage every second as if on fire, and eventually will die if it hasn't moved off the carpet.