A merged block is a reshape with a specific danger: it hides a checkout, and the checkout is where the clean happens. Lemon handles it with three guards. A tracked booking's own UID is never allowed to widen over another stay; a normal-length tracked reservation whose block balloons past the 30-night owner-closure threshold is routed to the reshape guard instead of being skipped; and nights inside the block that no tracked booking explains are surfaced to the host as uncovered nights under a stable synthetic identifier keyed on the block's checkout.
The incident that produced the third guard (INC-5, September 2026) was a 5-night stay and a 30-night stay exported as one 35-night block.