dpm breaks only if used in form custom submit on edit node
Thumbson was spot on. MySQL was hitting it's the packet limit and increasing maximum_allowed_packet in /etc/mysql/my.cnf solved the problem. (ubuntu path)

The error won't show in Drupal, the only place I could find it was in my php-fpm log. See below:

[13-Jan-2015 00:54:27] WARNING: [pool www1] child 21620 said into stderr: "NOTICE: PHP message: PHP Fatal error:  Uncaught exception 'PDOException' with message 'SQLSTATE[HY000]: General error: 2006 MySQL server has gone away' in /srv/w$
[13-Jan-2015 00:54:27] WARNING: [pool www1] child 21620 said into stderr: "Stack trace:"
[13-Jan-2015 00:54:27] WARNING: [pool www1] child 21620 said into stderr: "#0 /srv/www/eslsearch.net/public/includes/database/database.inc(2171): PDOStatement-&gt;execute(Array)"
[13-Jan-2015 00:54:27] WARNING: [pool www1] child 21620 said into stderr: "#1 /srv/www/eslsearch.net/public/includes/database/database.inc(683): DatabaseStatementBase-&gt;execute(Array, Array)"
[13-Jan-2015 00:54:27] WARNING: [pool www1] child 21620 said into stderr: "#2 /srv/www/eslsearch.net/public/includes/database/database.inc(2350): DatabaseConnection-&gt;query('SELECT expire, ...', Array, Array)"
[13-Jan-2015 00:54:27] WARNING: [pool www1] child 21620 said into stderr: "#3 /srv/www/eslsearch.net/public/includes/lock.inc(167): db_query('SELECT expire, ...', Array)"
[13-Jan-2015 00:54:27] WARNING: [pool www1] child 21620 said into stderr: "#4 /srv/www/eslsearch.net/public/includes/lock.inc(146): lock_may_be_available('schema:runtime:...')"
[13-Jan-2015 00:54:27] WARNING: [pool www1] child 21620 said into stderr: "#5 /srv/www/eslsearch.net/public/includes/bootstrap.inc(433): lock_acquire('schema:runtime:...')"
[13-Jan-2015 00:54:27] WARNING: [pool www1] child 21620 said into stderr: "#6 /srv/www/eslsearch.net/public/includes/bootstrap.inc(455): DrupalCacheArray-&gt;set(Array)"
[13-Jan-2015 00:54:27] WARNING: [pool www1] child 21620 said into stderr: "#7 [internal function]: DrupalCacheArray-&gt;__destruct()..."