At least on the .45 cal. model, the main recoil spring can be installed backwards, eventually locking up the slide.  The opened end of the spring goes forward into the slide, the "flat" end goes against the "nail head" of the recoil rod. Installing it backwards, the opened end of the spring begins to screw itself over the "nailhead" of the recoil rod. An easy repair, yes, but you don't want it to happen, especially at a critical moment. (it happened while at the range)