That a saving reappears later as rework, in which case it was never a saving but a deferred cost at a worse exchange rate. Each proposal is set against the settled requirements it would break, flagged at the point of decision rather than found in a later review. On a real sequence, a cladding substitution undid two settled requirements and surfaced as a claim 22 weeks later.

See how Tektome handles this in Commercial & Change Control