What broke
Two sessions were rewriting the same two landing pages whole, each from its own copy, and one copy was always behind. The same eight changes were on main at 23:51, gone at 23:56, back at 00:31, gone again at 00:32. Neither session saw a conflict, because neither merged: each replaced the file. The work was lost twice in forty minutes and nothing signalled it.
The rule it paid for
One file, one writer. A change on another writer’s file is asked for by message, never made in passing. No file is rewritten whole from a copy older than the remote head; a transformation is replayed on the head, and an anchored replay fails loudly on exactly the lines the other writer changed.