Document recovery behavior after interrupted multi-parity sync in Known Issues

doc/snapraid.txt

@@ -2045,6 +2045,36 @@ To avoid this limitation, avoid modifying protected files through hard
 links that are excluded from SnapRAID, or ensure that all hard links to a
 file are included in the scan so SnapRAID can normalize them.
 
+16.8 Recovery after an interrupted multi-parity sync
+----------------------------------------------------
+
+In an array with multiple parity disks, an interrupted `sync` can leave
+parity levels temporarily out of sync with each other. For example,
+parity 1 might already contain newly updated data, while parity 2 still
+contains the old data.
+
+During `check` or `fix`, SnapRAID tries different parity combinations
+and validates reconstructed data against the known block hashes (covering
+the full block, typically 256 KiB).
+
+If SnapRAID attempts a reconstruction using an inconsistent mix of old
+and new parity, the linear RAID equations will produce incorrect data.
+For this invalid combination to be falsely accepted, the equations would
+have to accidentally reconstruct at least one complete 256 KiB block so
+perfectly that its hash matches, while other unsynchronized blocks in the
+same stripe are reconstructed with incorrect data.
+
+This scenario does not depend on a hash collision; rather, the mixed
+parity equations themselves would have to coincidentally yield the exact
+original 256 KiB block contents. While mathematically possible with
+specially crafted or highly repetitive data, it is practically impossible
+with real-world data.
+
+If all disks are healthy, completing the interrupted `sync` before making
+further changes removes this ambiguity. If a disk has failed and recovery
+is required, avoid modifying any data disks before running `check` or
+`fix`.
+
 
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