diff --git a/remotefilelog/debugcommands.py b/remotefilelog/debugcommands.py
--- a/remotefilelog/debugcommands.py
+++ b/remotefilelog/debugcommands.py
@@ -201,8 +201,8 @@
     for path in paths:
         if '.data' in path:
             path = path[:path.index('.data')]
-        ui.write("%s:\n" % path)
         dpack = datapack.datapack(path)
+
         node = opts.get('node')
         if node:
             deltachain = dpack.getdeltachain('', bin(node))
@@ -216,12 +216,9 @@
             hashformatter = short
             hashlen = 14
 
-        lastfilename = None
         totaldeltasize = 0
         totalblobsize = 0
         def printtotals():
-            if lastfilename is not None:
-                ui.write("\n")
             if not totaldeltasize or not totalblobsize:
                 return
             difference = totalblobsize - totaldeltasize
@@ -236,11 +233,20 @@
                 deltastr
             ))
 
+        lastfilename = None
+        bases = {}
+        nodes = set()
+        failures = 0
         for filename, node, deltabase, deltalen in dpack.iterentries():
+            bases[node] = deltabase
+            if node in nodes:
+                ui.write(("Bad entry: %s appears twice\n" % short(node)))
+                failures += 1
+            nodes.add(node)
             if filename != lastfilename:
                 printtotals()
                 name = '(empty name)' if filename == '' else filename
-                ui.write("%s:\n" % name)
+                ui.write("\n%s:\n" % name)
                 ui.write("%s%s%s%s\n" % (
                     "Node".ljust(hashlen),
                     "Delta Base".ljust(hashlen),
@@ -249,6 +255,7 @@
                 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:
@@ -262,10 +269,49 @@
                 hashformatter(deltabase),
                 str(deltalen).ljust(14),
                 blobsize))
-
         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