Thanks Navaria and dixiedragon, that is helpful! All the pictures of overheating I have been seeing usually had noticeable separation but went back together. They just looked gelled to my untrained eye and they did not volcano, so I had not put it all together.
Gel happens any time the soap becomes warm enough to turn into a translucent jelly-like paste. I think some people think the gel stage is only found during saponification, but soap will become a gel any time it reaches the right temperature to turn into a gel. The "right temperature" for a given soap depends on the fatty acids and water content in the soap. Gel often happens during saponification of CP soap, but it can also happen during the "cook" when making HP soap, or even when doing a rebatch by heating soap scraps in a crock pot.
Overheating is when so much heat is generated by saponification that the soap starts to grow in size due to expansion from the heat and to water vapor and other gases being created. At best, an overheated soap will develop cracks on the top. At worst, the soap can expand so much and so violently it will overflow out of the mold (volcano). Soap on its way to overheating does become hot enough to turn into a gel, but that's kind of incidental.
I've noticed that soaps that overheated but didn't volvano or crack had rough surface textures compared to my normal gelled soaps. I couldn't figure out what was going on at first. When I finally realized it, I made sure my oven was off before I put the soap in there and I don't put anything around the molds any more. I just put a "shower cap" looking bowl cover over them and leave them overnight. It's only happened on time since I started doing that and that was when I did all of the butters and goat's milk. I think it was the milk.
I have had it split in the center, actually volcano with "boiling" areas that remain in the mold, and spew. All in different batches. I have learned that when I see liquid appearing on the top with a sandy appearance underneath, I need to get it into an ice bath, FAST. This is why I do not leave the house for several hours after making soap, I stay home and check that soap on a timed schedule.