If you excuse plain crappy optics that are known for high failure rates and focus on the 'duty rated' optics, I have seen a few interesting and repeatable failures trending. First, as mentioned above the SRO and RMRHD (overhanging) I have seen stoppages like the one Ben shared today. I have seen countless RMR's go downrange, based on speaking to the owners of said projectiles, I can only assume it was user-induced failure due to incorrect torque on the screws. All were sheered off.
Moving only other failures: open emitters, this has been discussed but they can become occluded and sometimes the fix of the weapon system cycling does not always solve the problem. One interesting failure I shared back in January here was one of our team members exhaling while holding the pistol at a high compressed ready (RMR type2) and the fog from his exhale complete occluded the emitter until it naturally dissipated. This was in a times shoot-house run and he simply had to wait or fire on point shooting only, could not even see his irons.
I have seen some slide cycling issues likely due to a lighter recoil spring not being able to account for the added weight of the optic, those are simple fixes to resolve during the build of the weapon system in my opinion.
Countless optics have gone down in classes due to batteries dying, but again, that is a user induced problem.