Made a simple redstone pulse generator using the 'one torch, one repeater, 6 dust' design from the wiki, and surrounded it with a wooden fence. The pulser design was one that burns out if the repeater delay is shorter than 2. I set the delay to 4 and linked it to a dispenser and the setup worked fine for many game days. Then some chickens got trapped inside the circuitry, and I ignored them for some time, but after a while I found the pulser had burned out. I removed the torch and reset the repeater to a delay of 4 (but left the chickens there to see if they really were the culprits), and after a few days they seemed to manage it again?! Is it possible/intentional for a chicken to be able to bump a redstone repeater and change its setting?
After further time I've also had the circuit burn out merely when I've travelled many blocks away, so this may be an unloaded chunks issue rather than a mob issue.