diff --git a/redo-ifchange.py b/redo-ifchange.py
index 63c2bc4..d178c71 100755
--- a/redo-ifchange.py
+++ b/redo-ifchange.py
@@ -28,9 +28,16 @@ def dirty_deps(f, depth, max_changed):
     if not f.stamp:
         debug('%s-- DIRTY (no stamp)\n' % depth)
         return DIRTY
-    if f.stamp != f.read_stamp():
-        debug('%s-- DIRTY (mtime)\n' % depth)
-        return DIRTY
+    newstamp = f.read_stamp()
+    if f.stamp != newstamp:
+        if newstamp == state.STAMP_MISSING:
+            debug('%s-- DIRTY (missing)\n' % depth)
+        else:
+            debug('%s-- DIRTY (mtime)\n' % depth)
+        if f.csum:
+            return [f]
+        else:
+            return DIRTY
 
     must_build = []
     for mode,f2 in f.deps():
diff --git a/t/stamp/.gitignore b/t/stamp/.gitignore
index 4e5c0b5..ae5524f 100644
--- a/t/stamp/.gitignore
+++ b/t/stamp/.gitignore
@@ -3,3 +3,4 @@
 /stampy
 /inp
 /bob
+/usestamp2
diff --git a/t/stamp/clean.do b/t/stamp/clean.do
index fac22fd..3bdd118 100644
--- a/t/stamp/clean.do
+++ b/t/stamp/clean.do
@@ -1 +1 @@
-rm -f *.log usestamp stampy inp bob *~ .*~
+rm -f *.log usestamp usestamp2 stampy inp bob *~ .*~
diff --git a/t/stamp/stamptest.do b/t/stamp/stamptest.do
index d5f973b..8b98df0 100644
--- a/t/stamp/stamptest.do
+++ b/t/stamp/stamptest.do
@@ -1,10 +1,12 @@
-rm -f stampy usestamp stampy.log usestamp.log
+rm -f stampy usestamp usestamp2 stampy.log usestamp.log usestamp2.log
 echo one >inp
 
 ../flush-cache.sh
 redo stampy
 [ "$(wc -l <stampy.log)" -eq 1 ] || exit 11
 
+# stampy already exists, so we won't generate it a second time, even though
+# usestamp depends on it.
 redo-ifchange usestamp
 [ "$(wc -l <stampy.log)" -eq 1 ] || exit 21
 [ "$(wc -l <usestamp.log)" -eq 1 ] || exit 12
@@ -14,27 +16,43 @@ redo stampy
 [ "$(wc -l <stampy.log)" -eq 2 ] || exit 31
 [ "$(wc -l <usestamp.log)" -eq 1 ] || exit 32
 
+# same as above: stampy is already up-to-date, so it won't be redone.
 redo-ifchange usestamp
 [ "$(wc -l <stampy.log)" -eq 2 ] || exit 41
 [ "$(wc -l <usestamp.log)" -eq 1 ] || exit 42
 
+# stampy depends on bob, so we'll have to rebuild stampy automatically.  But
+# stampy's checksum will still be identical.
 ../flush-cache.sh
 redo bob
 redo-ifchange usestamp
 [ "$(wc -l <stampy.log)" -eq 3 ] || exit 43
 [ "$(wc -l <usestamp.log)" -eq 1 ] || exit 44
 
+# Make sure the previous step correctly marked stampy and usestamp as up-to-date
+# even though *neither* of them is newer than bob.
 ../flush-cache.sh
 redo-ifchange usestamp
 [ "$(wc -l <stampy.log)" -eq 3 ] || exit 45
 [ "$(wc -l <usestamp.log)" -eq 1 ] || exit 46
 
+# now we're changing the contents of stampy.
 ../flush-cache.sh
 echo two >inp
 redo stampy
 [ "$(wc -l <stampy.log)" -eq 4 ] || exit 51
 [ "$(wc -l <usestamp.log)" -eq 1 ] || exit 52
 
-redo-ifchange usestamp
+redo-ifchange usestamp usestamp2
 [ "$(wc -l <stampy.log)" -eq 4 ] || exit 61
 [ "$(wc -l <usestamp.log)" -eq 2 ] || exit 62
+[ "$(wc -l <usestamp2.log)" -eq 1 ] || exit 62
+
+# when we delete the file and it gets regenerated identically, it's as good as
+# never having been deleted.  So usestamp won't need to change.
+../flush-cache.sh
+rm -f stampy
+redo-ifchange usestamp usestamp2
+[ "$(wc -l <stampy.log)" -eq 5 ] || exit 71
+[ "$(wc -l <usestamp.log)" -eq 2 ] || exit 72
+[ "$(wc -l <usestamp2.log)" -eq 1 ] || exit 73
diff --git a/t/stamp/usestamp2.do b/t/stamp/usestamp2.do
new file mode 100644
index 0000000..e338099
--- /dev/null
+++ b/t/stamp/usestamp2.do
@@ -0,0 +1,3 @@
+redo-ifchange stampy
+echo 2 $$ >>usestamp2.log
+cat stampy