diff --git a/TestGrid.md b/TestGrid.md
index 9bcb86f..e1374ee 100644
--- a/TestGrid.md
+++ b/TestGrid.md
@@ -29,6 +29,8 @@ probably out of date and inaccurate. #13 and other tickets are where we're
 keeping track of better grid-wide instrumentation tools that we want to
 build.
 
+**These measurements have been broken by changes to the Tahoe source code.**
+
 ### count of directories in the grid
  * [/tahoe-munin/tahoecs2.allmydata.com-tahoe_dirnodes.html other time scales]
 ![](http://allmydata.org/tahoe-munin/tahoecs2.allmydata.com-tahoe_dirnodes-month.png)