We had a guy at my last company who always seemed a bit ... dull. Not dumb, just slow and kind of out of it, fine via email but not very good at explaining simple things or having a conversation longer than two minutes. He would tell me he couldn't fix errors when I knew he could, he just didn't want to go to the trouble.
He was "leaving the company" about 6 months after I started (he'd been there at least 3-4 months longer than me, perhaps more than that), and he was being pretty unclear about what he was leaving for. Most people I worked with were hotshots and would leave for great jobs at great companies.
It came out at a company dinner on his last day, from his BOSS, who had clearly had more than enough, that he was consistently, daily, somewhere between tipsy and plastered. First thing in the morning it was usually tipsy, after lunch it was significantly worse. There may have been alcohol in his desk. They came to "a mutual agreement that he'd seek other opportunities."
I have no idea why he was able to stay there for more than 9 months, but it did explain one thing I'd been pretty curious about: Why did he always smell of whiskey?