diff --git a/mercurial/transaction.py b/mercurial/transaction.py
--- a/mercurial/transaction.py
+++ b/mercurial/transaction.py
@@ -54,6 +54,10 @@
             checkambig = checkambigfiles and (f, '') in checkambigfiles
             try:
                 fp = opener(f, 'a', checkambig=checkambig)
+                if fp.tell() < o:
+                    raise error.Abort(_(
+                            "attempted to truncate %s to %d bytes, but it was "
+                            "already %d bytes\n") % (f, o, fp.tell()))
                 fp.truncate(o)
                 fp.close()
             except IOError: