I signed off ESA because I was earning above the disregard, but only working 4-6 hours a week, just before my Tribunal was due to be heard. They managed to send me two letters - one saying I was signing off because I was working 16+ hours, and one to say due to the change of circumstances, the Tribunal had met and disallowed my appeal! (it hadn't met - and thanks to B&W for suggesting I check with the Tribunal if they were still expecting me at the hearing!)
I got my MP involved.
I've had a lovely letter of apology and explanation, and been told they will make me a "consolatory special payment" for the distress.
I have just sent them a response escalating the complaint (because although they were apologetic, they weren't going to do anything about their computer glitches or put in place a system that reduces the risk of human glitches).
So, big thanks for the encouragement and info from the site. I will let you know if/when they find a way to let their computer code someone as closing a case due to going over the disregard.