A reply from the end-of-war thread, that I refrained from posting after your note:
Lots of things, actually. If the design specs call for something to be 99.3% reliable, you’ll probably be able to achieve that… but if the design specs call for it to be 99.999% reliable, you’ll probably get about 98%. And a lot of military hardware is especially prone to those over-optimistic estimates, especially given that a realistic assessment of military-hardware reliability in actual use is 50%, given that in any engagement, one side’s going to lose.