There is s certain type of general anaesthetic which is very painful.

I had it for the last procedure I went through. They warned me it would hurt and I didn't take it seriously because I've never had problems before. It felt like my veins were aching and on fire but at that point I was too far gone to say or do anything about it.

I think this is what people who wake up from lethal injection gone wrong are complaining about. It's not that bad, you have to remember crims love playing the victim.