I generally cut ill people a lot of slack, because I feel better about being cranky and annoying when I'm ill myself then.
However, I won't excuse stupidity on a major scale because of illness. Say someone bought something shiny they couldn't possibly afford because they had a cold and needed cheering up. And if you're unable to fulfil obligations because of illness, then you should do all you can to fix a replacement or minimise the damage.
How about concussion or shock? I know I've done some pretty stupid things while freshly concussed. Like going skiing. Or leading a board meeting just minutes after having a pile of metal junk dropped on my head. Worst board meeting ever. There was a full-blown mutiny that I didn't even
notice until days later when the original chair called me and demanded to know what the hell I'd been up to.
I could've acted differently, but at the time I was only able to hold on to one thought at a time, and the thought that stuck was "lead the board meeting", not "lie down in a corner and whimper" which had been the better choice.
So, is headtrauma a valid excuse for stupidity? I had perfect control over my actions, but I wasn't able to evaluate all my options properly. I've never been severely depressed or so, but I think it's similar in a way. You have less options "accessible", so you don't always make the rational choice because you simply can't
think of it.