Epoxy cures through an exothermic (heat generating) reaction. If left to cure in a contained mass, (e.g., a mixing pot) it can generate enough heat to melt the plastic and burn your skin. It can also ignite any combustible materials nearby. The thicker (or larger) the epoxy mass, the more heat it produces, generating toxic vapors that include ammonia and carbon monoxide.
To prevent heat buildup, transfer the mixed epoxy from the mixing pot to a wide, shallow container (such as a mixing pan). When you need to fill large cavities with epoxy, do so in multiple layers instead of one thick layer.
Do not throw leftover mixed epoxy into a trash can. While it cures, set the container aside where you can keep an eye on it, and turn on a fan to help disperse the toxic vapors.