diff --git a/mercurial/obsolete.py b/mercurial/obsolete.py
--- a/mercurial/obsolete.py
+++ b/mercurial/obsolete.py
@@ -414,16 +414,16 @@
         data.extend(parents)
     totalsize = _calcsize(format)
     for key, value in metadata:
-        lk = len(key)
-        lv = len(value)
+        lk = min(len(key), 255)
+        lv = min(len(value), 255)
         data.append(lk)
         data.append(lv)
         totalsize += lk + lv
     data[0] = totalsize
     data = [_pack(format, *data)]
     for key, value in metadata:
-        data.append(key)
-        data.append(value)
+        data.append(key[:255])
+        data.append(value[:255])
     return ''.join(data)
 
 def _fm1readmarkers(data, off, stop):
diff --git a/tests/test-obsolete-bounds-checking.t b/tests/test-obsolete-bounds-checking.t
new file mode 100644
--- /dev/null
+++ b/tests/test-obsolete-bounds-checking.t
@@ -0,0 +1,17 @@
+Create a repo, set the username to something more than 255 bytes, then run hg amend on it.
+
+  $ cat >> $HGRCPATH << EOF
+  > [ui]
+  > username = aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa <very.long.name@example.com>
+  > [extensions]
+  > amend =
+  > EOF
+  $ hg init tmpa
+  $ cd tmpa
+  $ echo a > a
+  $ hg add
+  adding a
+  $ hg commit -m "Initial commit"
+  $ echo a >> a
+  $ hg amend
+  saved backup bundle to $TESTTMP/tmpa/.hg/strip-backup/78a4d7b136df-b05e3630-amend.hg (glob)