docs: known issue for the flapping bad_nbit_parms_walk classification

The committed CONFORMANCE.md counts bad_nbit_parms_walk.h5 as an
our-error because its six h5py reads agreed in that run; a rerun the
same day confirmed the over-read and counted it ref-bug. The ok count
(602 of 697) is the same either way.

Co-Authored-By: Claude Opus 5.5 (1M context) <[email protected]>

docs/known-issues.md

@@ -23,6 +23,7 @@ Checked against `main` at `9b5803f` on 2026-09-28.
 | [Range reads (`File::open_storage`) limits](#range-reads-fileopen_storage-limits) | cost; zero-copy APIs need an in-memory file | 2026-09-26 |
 | [Remote files (`clawhdf5-remote`) limits](#remote-files-clawhdf5-remote-limits) | untested backends, fixed block size, timeouts | 2026-09-26 |
 | [`clawhdf5-wasm` (browser) limits](#clawhdf5-wasm-browser-limits) | memory, round trips, unsupported types | 2026-09-26 |
+| [Conformance: `bad_nbit_parms_walk.h5` flips between ref-bug and our-error](#conformance-bad_nbit_parms_walkh5-flips-between-ref-bug-and-our-error) | report noise (ok count unaffected) | 2026-09-28 |
 | [The Node.js package does not work](#the-nodejs-package-packagesclawhdf5-node-does-not-work) | broken, unpublished, not in CI | 2026-09-25 |
 
 ---
@@ -378,6 +379,26 @@ browser (`clawhdf5-wasm`'s `openUrl`) open URLs since 2026-09-27.
 - External links and virtual-dataset sources in other files cannot be
   followed (no file system).
 
+## Conformance: `bad_nbit_parms_walk.h5` flips between ref-bug and our-error
+
+**Status:** open (found 2026-09-28). Report noise, not a clawhdf5 bug: the
+ok count (602 of 697) and the gate are unaffected.
+
+`hdf5/test/testfiles/bad_nbit_parms_walk.h5` `/Nbit_int_data_le` is an
+object clawhdf5 refuses and HDF5 2.0 reads only by over-reading memory (see
+[the last non-ok files](#conformance-the-last-non-ok-files)).
+`conformance/ref_bugs.py` counts it as *ref-bug* only when h5py's values
+differ across its six differently set-up reads. The committed
+`CONFORMANCE.md` (run of 2026-09-28 04:29 UTC, `bf5a163`) saw one distinct
+result in six reads, so it reports the file as **1 our-error** and 2
+ref-bug; a rerun on tank the same day (16:06 UTC, `conformance/run.sh
+--no-fetch`, docs-only changes on top of `9b5803f`) saw three distinct
+results and reports 0 our-error, 3 ref-bug. Whether libhdf5's over-read
+changes between processes depends on heap layout, so six reads do not
+always expose it. A fix belongs in `conformance/ref_bugs.py` (more or more
+varied reads for this object) or in documenting the file as a known
+refusal; neither is done.
+
 ## The Node.js package (`packages/clawhdf5-node`) does not work
 
 **Status:** open (found 2026-09-25; re-checked 2026-09-28, unchanged).