I have witnessed so many brain fart d'oh moments I could write a book. For example.....
My old boss called me in on a Sunday to help him fix his aerator. He said he had been working on it for several hours and couldn't figure out why it wouldn't start. I show up, and it is completely dismantled. He took out the engine, the carburetor, the fuel lines, literally gutted and cleaned the entire thing, put it back together, then took it apart again when it still wouldn't work. So here it was, in a million pieces, and my job was to try and figure out which part was the faulty part.
I noticed the problem immediately, but rather than point it out, I figured I'd have some fun at his expense. I said uh, huh, I have a hunch - then I started to put it all back together. When i was finished, he asked me what the hell I was going to accomplish by rebuilding it without changing anything. It was at that point that I flipped the switch into the RUN position and said go ahead old man, give it a try, something tells me we've got this thing licked.
I fell to the ground beet red and laughing hysterically while he shook his head in disbelief and nearly cried before he started laughing too. This poor bastard was 78 years old at the time, and had worked on this damn thing for 5-6 hours, completely wearing himself out. All because he didn't turn it on and hadn't used it in so long that it never occured to him.