Things can happen by accident when you're being violently attacked. I slapped DS across the face once when he was having a meltdown. He'd knocked my glasses off so I couldn't see well and had then punched me in the eye which caused double vision, I just flung my arm out as I could see him pulling back to punch me again, misjudged it and slapped him. DS has no restraint, no sense of self preservation, no holding back when he has a meltdown. He will bite, kick, pull hair, try to gouge lumps out of us with his nails, headbutt, punch, slap, use anything he can get his hands on as a weapon, all the while screaming at us that he wants to kill us, that he wants to die, that we should kill him, that he hates us.
We have asked and asked for help with his meltdowns and were lent a booklet on NVR, which is bloody useless once he's tipped over into having a meltdown, is attacking us and trying to hurt himself. I've had three trips to A&E, DH has recieved multiple blows to the head, we've both been left bruised and bleeding - at one point pretty much my entire right arm was a massive bruise, as was my back. No real help, except the suggestion to call the police, the police won't come out, 999 say the two of us should be able to manage him and we need to do 'whatever it takes'.
To the person that asked what happens as they get older - they get harder to manage safely. DS has improved as he's had help with learning how to regulate his emotions through school and specialist courses, but it's not perfect and sometimes he gets overwhelmed, and then just asking him not to do something can tip him over.
He's 11 now and I can't manage him safely alone if he has a meltdown. We try to have the two of us here as much as possible. I don't know what happens if he still has meltdowns at 14, 15, 16, and we've still had no help.