diff --git a/docs/specs/qcow2.txt b/docs/specs/qcow2.txt
index 87bf785..339cdc1 100644
--- a/docs/specs/qcow2.txt
+++ b/docs/specs/qcow2.txt
@@ -75,7 +75,12 @@ in the description of a field.
                     Bitmask of incompatible features. An implementation must
                     fail to open an image if an unknown bit is set.
 
-                    Bits 0-63:  Reserved (set to 0)
+                    Bit 0:      Dirty bit.  If this bit is set then refcounts
+                                may be inconsistent, make sure to scan L1/L2
+                                tables to repair refcounts before accessing the
+                                image.
+
+                    Bits 1-63:  Reserved (set to 0)
 
          80 -  87:  compatible_features
                     Bitmask of compatible features. An implementation can