So on a 1911, the thumb safety pretty literally clamps the sear to the hammer. The only way it's going to go off is if the sear literally crumbles/breaks; even then since the sear is being clamped to the hammer, the hammer will rotate down to the safety notch/"half cock" where it'll be stopped by the remanent of the sear. This is why it's imperative that the thumb safety actually be well fit to the hammer/sear combo, the detent spring that holds the thumb safety up be in good repair, and the notch in the thumb safety that the detent rests in be well defined.
So long as the thumb safety is fit correctly, when the thumb safety is engaged, there's pretty much no way of inducing an accidental discharge via parts failure unless an extreme set of circumstances pops up (like the sear and thumb safety both crumble to dust at the same time).
This, btw, is also why I've settled on not carrying a 1911 unless it's a Colt Series 80, with it's firing pin safety. Kimber's implementation of the Swartz safety is pretty terrible and pretty easily mistimed out of the factory, and Smith's setup isn't much better since they also rely on the grip safety deactivating the firing pin safety. Colt's setup times it off of the trigger...and while it can be mistimed, it's also a fairly easy fix to understand and implement. C&S even sells the appropriate bits. While I have faith that the thumb safety will correctly do it's job, having the firing pin safety adds that little extra peace of mind...not to mention making it a whole lot more drop safe.