Over the years I have worked with some dumb people, but I think the dumbest I ever worked with was Frank.  Frank wasn't retarded, had actually gotten through high school!  But he was just plumb stupid. He could not follow directions, and would do things without thinking.
One day he was prepping some food and complained the knife wasn't sharp enough.  So one of the cooks sharpened it for him.  Frank looked at the knife, went "Oh, it's not THAT sharp!"  and proceeded to draw it down a rag he had wrapped around his finger.  The result was they had to rush him to the emergency room for stitches.  But what FINALLY made us send Frank on his way, was that no matter how many times you told him not to, he'd mix cleaning chemicals together.
I had been downstairs getting stock one morning when I came upstairs in the elevator and the minute the doors opened I just about suffocated!  I rushed into the kitchen to find everybody staggering around and to see vapor rising out of Frank's mop bucket.  I grabbed his bucket, shoved it into the elevator, and sent it downstairs, cleared everybody out of the kitchen, then put the exhaust fans on full, telling everybody to stay out of the kitchen until I told them to return.
The manager came out and asked me what was wrong.  I said "I'm not sure, but I think Frank mixed ammonia and bleach.  The kitchen's full of gas!"
The manager stepped into the kitchen a moment, came back out and said "Wooo!  You're right!  Let it clear.  Don't let anybody in there until you think it's safe!"
Sure enough, I was right.  Frank had mixed the regular floor cleaning detergent, about half a bottle of bleach, about half a bottle of ammonia, and a gallon or so of water!  He thought this concoction would REALLY clean the floors!
I don't know if it would have killed anybody, but it sure as heck made everybody in the kitchen sick.  Everybody was complaining of symptoms for two or three days.
Not withstanding, that was the last day that Frank worked for us.  He couldn't understand but we had to let him go, as there was no telling what he would do next.
After that the ammonia was locked in my supply cabinet at all times, and I was the only one that was allowed to use it, or, I directly supervised whoever used it.
I will say one thing...the mop bucket Frank had used looked brand new.  The inside shone like new metal!  But I don't think I'd recommend anyone cleaning them that way!