diff --git a/docs/known-issues.md b/docs/known-issues.md
index 43f945c..9c5bf01 100644
--- a/docs/known-issues.md
+++ b/docs/known-issues.md
@@ -15,6 +15,7 @@ Checked against `main` at `9b5803f` on 2026-09-28.
 
 | Issue | Kind | Since |
 |---|---|---|
+| [NetCDF-4: an unlimited dimension reports size 0](#netcdf-4-an-unlimited-dimension-reports-size-0) | **wrong metadata** (dimension size; variable shapes and values are right) | 2026-09-28 |
 | [In-place modification (`FileEditor`) limits](#in-place-modification-fileeditor-limits) | refused edits (`Error::Unsupported`), space reuse per editor, no journal | 2026-09-26 |
 | [Python in-place editing limits](#python-in-place-editing-clawhdf5filepath-r-limits) | refused writes (`NotImplementedError`), deliberate conversion differences | 2026-09-27 |
 | [Selection reads that decode more than the selection](#selection-reads-that-decode-more-than-the-selection) | speed only | 2026-09-26 |
@@ -399,6 +400,21 @@ 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.
 
+## NetCDF-4: an unlimited dimension reports size 0
+
+**Status:** open (found 2026-09-28 while verifying the README refresh).
+`clawhdf5-netcdf4`'s `NetCDF4File::dimensions()` reports an unlimited
+dimension's `size` as 0 when variables along it hold records. Reproducer: with
+netCDF4-python, create dimension `time` (unlimited) and `x` (3), a variable
+`t(time, x)`, and write 2 records; netCDF4 reports `time` = 2 and `t` shape
+(2, 3). clawhdf5-netcdf4 reports `dim time size 0 unlimited true`, while
+`variable("t").shape()` correctly gives `[2, 3]`. In NetCDF-4 an unlimited
+dimension's length is the largest extent of the variables that use it (its
+dimension-scale dataset is not extended by netCDF-C), and the size is read
+from the dimension scale instead. Variable shapes and values are correct;
+only `Dimension::size` of unlimited dimensions is wrong. Workaround: use the
+variables' shapes.
+
 ## The Node.js package (`packages/clawhdf5-node`) does not work
 
 **Status:** open (found 2026-09-25; re-checked 2026-09-28, unchanged).