Keep an eye on G&P pistons. I find them to be a little brittle. Also I've had issue with the spur gear and sector gear where a tooth would break off and jam up the mechbox. If you were lucky, it'd fall away and get stuck elsewhere in the box and run just fine. No luck for me.
Here's a picture of the shattering piston. Only way I could get it to show on camera was with unique lighting.
