The exclude that used to be true
- date
- 20260818
- what happened
- A Windows backup plan excluded a user-profile path from its nightly backup, correctly, because at the time that path held an abandoned local profile. Months later, renaming the live 91.6 GB working profile onto that same path would have silently dropped the entire working profile from every nightly backup, with a green status and no error.
- what it cost
- Nothing. It was caught before the rename happened, and only because an unrelated question about a filesystem junction forced someone to read the plan's exclude list.
- what changed
- A rule: before renaming or reusing any filesystem path, grep every backup plan's excludes for that exact path first, because an exclude written for an old use of that path keeps matching silently once the path means something else. The configuration stored the path with backslashes while the display interface normalised it to forward slashes, so a naive search for the obviously correct form would have found nothing and reported false confidence.
- the check now
- Grep backup configurations for both slash forms of any path being renamed or repurposed, and verify the fix by snapshot content after the change rather than by job status, since a green status is exactly what the bug would also produce.
A backup exclude list is not really a list of paths. It is a list of claims about what those paths currently contain, and those claims silently go stale the moment a path is reused for something else. An exclude written correctly for an abandoned profile folder would have become, unnoticed, an exclude for an entire active working profile the day that folder was renamed onto the same location. The backup job would have kept reporting success the whole time, because from the tool’s point of view nothing had gone wrong. It was faithfully honouring an instruction that had simply stopped meaning what it once meant.
The near-miss was caught by accident. An unrelated question about how the operating system treats a filesystem shortcut forced someone to open the backup plan and actually read its exclude list before the rename went ahead.
The durable habit this produced is cheap and general. Before repurposing any path a backup system references, search every relevant configuration for that exact path, in every form it might be stored, and confirm the fix by checking that the content actually shows up in the next backup rather than trusting the job’s reported status. The two-slash-forms detail is the part most likely to bite. A tool can store one form and display the other, so a search for what you see on screen finds nothing.
Source: node0 lessons v0.1, lesson 3.11. Sanitized: checklist v0.1, 20260921; profile path generalised; voice pass 20260921. Part of oznog.com/node0.