Tile size would be recalculated constantly (every scene update) when the calculated size was less than 8 pixels (blender's own hard limit)
diff --git a/render_auto_tile_size.py b/render_auto_tile_size.py
index a773281..00f848e 100644
--- a/render_auto_tile_size.py
+++ b/render_auto_tile_size.py
@@ -20,7 +20,7 @@ bl_info = {
     "name": "Auto Tile Size",
     "description": "Estimate and set the tile size that will render the fastest",
     "author": "Greg Zaal",
-    "version": (3, 0),
+    "version": (3, 1),
     "blender": (2, 74, 0),
     "location": "Render Settings > Performance",
     "warning": "",
@@ -300,6 +300,12 @@ def do_set_tile_size(context):
     else:
         settings.threads_error = False

+    # Make sure tile sizes are within the internal limit
+    tile_x = max(8, tile_x)
+    tile_y = max(8, tile_y)
+    tile_x = min(65536, tile_x)
+    tile_y = min(65536, tile_y)
+
     render.tile_x = tile_x
     render.tile_y = tile_y