diff --git a/tests/README.md b/tests/README.md
index 4f4764cb7..14afe2472 100644
--- a/tests/README.md
+++ b/tests/README.md
@@ -79,3 +79,13 @@ TOTAL                                                      6714   1422    79%
 
 ============================ 79 passed in 5.76s ==============================
 ```
+
+## Known test failure with non-default build of MuPDF
+
+If PyMuPDF has been built with a non-default build of MuPDF (using
+environmental variable `PYMUPDF_SETUP_MUPDF_BUILD`), it is possible that
+`tests/test_textbox.py:test_textbox3()` will fail, because it relies on MuPDF
+having been built with PyMuPDF's customized configuration, `fitz/_config.h`.
+
+One can skip this particular test by adding `-k 'not test_textbox3'` to the
+pytest command line.