Create a repo, set the username to something more than 255 bytes, then run hg amend on it.

  $ unset HGUSER

  $ cat >> $HGRCPATH << EOF

  > [ui]

  > username = aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa <very.long.name@example.com>

  > [extensions]

  > amend =

  > [experimental]

  > evolution.createmarkers=True

  > evolution.exchange=True

  > EOF

  $ hg init tmpa

  $ cd tmpa

  $ echo a > a

  $ hg add

  adding a

  $ hg commit -m "Initial commit"

  $ echo a >> a

  $ hg amend 2>&1 | grep -E -v '^(\*\*|  )'

  transaction abort!

  rollback completed

  Traceback (most recent call last):

  *ProgrammingError: obsstore metadata value cannot be longer than 255 bytes (value "aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa <very.long.name@example.com>" for key "user" is 285 bytes) (glob)