There's a scene - It might be in Alice In Wonderland, but somebody can surely say in comments - where a character meets a centipede (a talking one, of course) and says "Wow, how do you coordinate all those legs to walk?" The centipede thinks about it a minute, says "Well gee, I don't know!" and thereafter, can't walk again, since he's now thinking about it instead of just doing it.
Another reference is the book Snow Crash, by Neal Stephenson, which centers around a 'virus', which consists of basically a snowy video screen. The particular pattern in the snow sends a message into the brain of anyone watching it, basically disabling their mind.
Sometimes, I think about how I'm breathing, and I have to try to distract myself, so I'm not doing that anymore! Once you start thinking about it, you have to actually manually control it until you can stop thinking about it. I don't think there's actually any danger there, but it's sort of annoying when you get too focused on it.
Other times, I get lost in thought, and my mind starts wheeling down strange lines, into thinking about thinking, and I get to a point where I'm wondering how I'm thinking. I know the mind is effectively a computer - in fact, I've actually experienced a real live reboot once after I fainted (my life literally flashed before my eyes, random memories one scene after another, faster and faster, until the real world faded back in). A strange experience, but a real awakening one! So this computer, like any, can get caught in loops and can crash from bad data. Luckily, as a biological computer, it's extremely parallel, offering lots of backup and circumventing of problems. But still, sometimes I'll sit and think about something and wonder if there are actually some thoughts you can't have, because if you do, they'd shut your brain down, or trap it in a loop. Maybe some people are crazy because of a thought they had. Wouldn't it be interesting if somebody discovered the great secret of the universe, only thinking about that breaks your brain?
An interesting example of this kind of thing is in this Mind Hacks
article. A guy looks at staircases and has strange specific seizures. That's some funny wiring. And certain types of thinking can cause some kinds of seizures. Brain=computer. I find neuroscience very fascinating. But I do worry that one day, I'm going to have the one wrong thought, and I'm just going to be gone. Scary notion? HAPPY HALLOWEEN!