Assertion failure from InnoDB while restoring using Innobackupex
Luckily there is a small fix for this if you do not have the luxury of stopping traffic on the DB. By using --ibbackup xtrabackup_55 the recovery recognizes the pending transactions and handles them accordingly.

Our script now looks like this:

nice -n 19 innobackupex-1.5.1 --stream=tar ./ --user XXXX --password 'YYYYY' \
  --ibbackup xtrabackup_55 | \
  nice -n 19 /usr/bin/pigz - | \
  ssh -p 32231 ***@backup.****.com \
    'cat - &gt; /var/backup/db/#{Time.now.strftime("%Y-%m-%d")}.tar.gz'


and 

innobackupex-1.5.1 --apply-log . --ibbackup xtrabackup_55


respectively.

Just thought I would share this tidbit, due to the fact that looking for a solution cost me approx. 3,5 hours.