Unusual software bugs
Thanks to fellow Access MVP Graham R Seach for showing that these kinds of bugs really do have a name. And that it's not just me.
"Unusual software bugs are a class of software bugs that are considered exceptionally difficult to understand and repair. There are several kinds, mostly named after scientists who discovered counterintuitive things."
Heisenbug, Bohrbug, Mandelbug and Phase of the Moon bug are among the bugs mentioned.