I had one of those, customer is adamant latest version broke some function, I check related code and it hasn't been touched for 7 years, and as written couldn't possibly work. I try and indeed, doesn't work. Yet customer persisted.
Long story short, an unrelated bug in a different module caused the old, non-functioning code to do something entirely different if you had that other module open as well, and the user had disciverdd this and started relying on this emergent functionality.
I had made a change to that other module in the new release and in the process returned the first module to its non-functioning state.
The reason they interacted was of course some global variables. Good times...
Which means 1. anything that gets through, almost by definition, is going to be bad enough to escape the safeguards, and 2. when things do get bad enough to escape the safeguards, it will likely expose the avalanche of things that were already in a failure state but were being mitigated
The takeaway, which I'm not really sure how to practically make use of, was that if a system isn't observably failing occasionally in small ways, one day it's going to instead fail in a big way
I don't think that's necessarily something rigorously proven but I do think of it sometimes in the face of some mess
