Second, although Heisenbugs may constitute the majority of software faults in some environments, our experience indicates that many network failures resulting in extensive system outages have been caused by rarely triggered Bohrbugs. As an extreme example, 99% Heisenbugs may contribute only 1% of total down time because they can be easily bypassed, while the remaining 1% Bohrbugs may cause 99% of total down time.

BOHR BUG n. A repeatable BUG; one which manifests reliably under a possibly unknown but well-defined set of conditions

The assertion that most production software bugs are soft Heisenbugs that go away when you look at them is well known to systems programmers. Bohrbugs, like the Bohr atom, are solid, easily detected by standard techniques, and hence boring.