I start off by getting twitchy, which turns into stimming (which I hope still just looks like "twitchy," but I certainly can't judge). If the over-stimulation continues for more than a few moments, the twitching/stimming stops and is replaced by a sort of derealization. Like jec6613, I become essentially nonverbal and have trouble thinking clearly. In fact, I have a lot of difficulty processing any information at all when over-stimulated; I have trouble understanding sounds and, especially, words (which reach me as mostly-meaningless sounds). Everything blurs into one poorly-processed cacophony of sound and light, while I am inside my body thinking, "You know what's going on. This is overload. Just focus." Which, of course, does not work in the slightest.