diff --git a/remotefilelog/debugcommands.py b/remotefilelog/debugcommands.py
--- a/remotefilelog/debugcommands.py
+++ b/remotefilelog/debugcommands.py
@@ -215,7 +215,6 @@
         hashformatter = short
         hashlen = 14
 
-    lastfilename = None
     totaldeltasize = 0
     totalblobsize = 0
     def printtotals():
@@ -233,7 +232,16 @@
             deltastr
         ))
 
+    lastfilename = None
+    bases = {}
+    nodes = set()
+    failures = 0
     for filename, node, deltabase, deltalen in dpack.iterentries():
+        bases[node] = deltabase
+        if node in nodes:
+            ui.warn("Bad entry: %s appears twice\n" % short(node))
+            failures += 1
+        nodes.add(node)
         if filename != lastfilename:
             printtotals()
             name = '(empty name)' if filename == '' else filename
@@ -246,6 +254,7 @@
             lastfilename = filename
             totalblobsize = 0
             totaldeltasize = 0
+
         # Metadata could be missing, in which case it will be an empty dict.
         meta = dpack.getmeta(filename, node)
         if constants.METAKEYSIZE in meta:
@@ -262,6 +271,46 @@
     if filename is not None:
         printtotals()
 
+    failures += _sanitycheck(ui, set(nodes), bases)
+    if failures > 1:
+        ui.warn("%d failures\n" % failures)
+        return 1
+
+def _sanitycheck(ui, nodes, bases):
+    """
+    Does some basic sanity checking on a packfiles with ``nodes`` ``bases`` (a
+    mapping of node->base):
+
+    - Each deltabase must itself be a node elsewhere in the pack
+    - There must be no cycles
+    """
+    failures = 0
+    for node in nodes:
+        seen = set()
+        current = node
+        deltabase = bases[current]
+
+        while deltabase != nullid:
+            if deltabase not in nodes:
+                ui.warn("Bad entry: %s has an unknown deltabase (%s)\n" %
+                        (short(node), short(deltabase)))
+                failures += 1
+                break
+
+            if deltabase in seen:
+                ui.warn("Bad entry: %s has a cycle (at %s)\n" %
+                        (short(node), short(deltabase)))
+                failures += 1
+                break
+
+            current = deltabase
+            seen.add(current)
+            deltabase = bases[current]
+        # Since ``node`` begins a valid chain, reset/memoize its base to nullid
+        # so we don't traverse it again.
+        bases[node] = nullid
+    return failures
+
 def dumpdeltachain(ui, deltachain, **opts):
     hashformatter = hex
     hashlen = 40