What broke
A file bridge between a cloud session and the developer machine answered written on two files it had not written, twice in one day. The modification time was fresh and the content was the old one. A commit prepared on those files would have published the previous version under a message describing the new one. The size caught it: the file on disk was 575 bytes short of the file that had been sent.
The rule it paid for
An acknowledgement is not a write. Every file put on a machine by a bridge is read back and compared to what was sent (size first, bytes after) before any command that depends on it is handed to the human. A second attempt goes by another route, never the same one again.