diff --git a/readme.md b/readme.md
index 352fd84..c25fad5 100644
--- a/readme.md
+++ b/readme.md
@@ -38,6 +38,12 @@ croppie.js:182 (mine)
 if (src.match(/^(file|https)?:\/\/|^\/\//)) {
 ```
 
+## known bugs
+
+* tile 0 (background colour only) is an implicit tile in Bitsy- it's not stored in the game data. so this tool ignores it. oops!!
+    * when importing game data, create a version of this
+    * when writing game data, don't write the 0 tile
+
 ## to do
 
 * allow user to save output as image, or tweet it :) *user can currently right-click -> Save As but the 128x128 size is not great*