Meltdowns caused directly by sensory and other discomforts that I didn't even have the words to articulate adequately, thus I couldn't explain why my fussiness was escalating by increments into full blown screaming and crying. There was a reason behind it, every time, but because my parents and everyone else around me couldn't see or hear the reason, because it wasn't something that bothered them (noisy restaurants, for one example) all they saw was a child who was "throwing a tantrum" for no discernible reason --- and therefore I must be just "bad" or even "possessed by the devil." Seriously, that was thrown around. I was shamed at a very early age.
Yet the thing was, between meltdowns caused by sensory overloads, I was actually a really sweet child when feeling calm and safe. But that was just it -- nobody, myself included, was clued in on what those "calm and safe" conditions were and that I needed them created when they were missing, or at least helped through coping when they couldn't be in place. This is why it's important to identify if there is autism, because then everyone can go "OHHHHHH, so now we know this coping thing is what helps, this other thing is what makes things worse" etc. Everyone was in the dark dealing with me in the 1960s.
Another big issue was of course social interaction. I was withdrawn most of the time, but if I did come out of my shell and interact, it was in a very one-sided way, monologing to a person instead of taking turns.
Something else I remember was being deadly afraid of someone knocking at the door. I would run and hide under furniture because (I've now figured out) I was scared by the uncertainty of not knowing who was there, coming into my world. To this day I feel creeped out like hell when someone knocks on my door and it isn't because I've either ordered pizza, bought from Amazon or invited a friend over. I hate unexpected door knocks.