Random-pattern-resistant fault
Beginner
A pretend flaw that random tests almost never catch, because it needs one very specific mix of inputs.
Novice
A fault that random patterns almost never detect because it needs one rare input combination, for example the output of an 8-input AND gate stuck at 0, which only shows when all eight inputs are 1.
Expert
Caps LBIST and random-first ATPG coverage. Fixed with weighted random patterns, deterministic top-up patterns, or test points that make the hard spot easier to control or observe.
Explained in Design for test (Design Flow).
See also: LBIST, Controllability and observability.