Raised by the Commitment-Floor Guard (Mitigation 1, spec §6/§16).
The catastrophic GVT bug is computing GVT too high, letting something commit and then roll back. The invariant that catches it: no LP may ever roll back or annihilate at a virtual time below the GVT it has been told is permanent. If GVT was ever too high, the next straggler below it trips this — converting a rare, silent, hard-to-reproduce corruption into a loud, immediate crash that fails the run with full diagnostics.