The failure mode a guardrail layer adds is the one where it quietly stops running. A validator that never fires and a validator that passes everything produce the same logs and the same green result, so the layer reads as working right up until someone checks. I lost real time on a memory search that kept returning junk because its ranking step was never being called at all, and nothing in the output said so. Worth having the ontology check emit a count of what it evaluated and not only what it rejected.