In D6 I used a series of node reference fields to select stories for inclusion in a newsletter, field_story01, field_story02, .... The edit form included 12 dropdowns to place stories in the desired order. I am now migrating to D7 and tried to recreate my newsletter content type. See http://drupal.org/node/268404#comment-3323958 for the approach I used.

All is well with up to 6 entity reference fields that perform the selection function. But after adding field_story07 I get the following errors.

Create Simplenews newsletter

    Add this newsletter issue to a newsletter by selecting a newsletter from the select list. To send this newsletter issue, first save the node, then use the "Newsletter" tab.
    Set default send options at Administration > Configuration > Web services > Newsletters.
    Set newsletter specific options at Administration > Content > Newsletters.

Additional uncaught exception thrown while handling exception.
Original

PDOException: SQLSTATE[HY000]: General error: 2006 MySQL server has gone away: SELECT * FROM {menu_router} WHERE path IN (:ancestors_0, :ancestors_1, :ancestors_2) ORDER BY fit DESC LIMIT 0, 1; Array ( [:ancestors_0] => user/autocomplete [:ancestors_1] => user/% [:ancestors_2] => user ) in menu_get_item() (line 465 of /home/spjdco5/public_html/spjdc.net/drupal7/includes/menu.inc).
Additional

PDOException: SQLSTATE[HY000]: General error: 2006 MySQL server has gone away: INSERT INTO {watchdog} (uid, type, message, variables, severity, link, location, referer, hostname, timestamp) VALUES (:db_insert_placeholder_0, :db_insert_placeholder_1, :db_insert_placeholder_2, :db_insert_placeholder_3, :db_insert_placeholder_4, :db_insert_placeholder_5, :db_insert_placeholder_6, :db_insert_placeholder_7, :db_insert_placeholder_8, :db_insert_placeholder_9); Array ( [:db_insert_placeholder_0] => 1 [:db_insert_placeholder_1] => php [:db_insert_placeholder_2] => %type: !message in %function (line %line of %file). [:db_insert_placeholder_3] => a:6:{s:5:"%type";s:12:"PDOException";s:8:"!message";s:295:"SQLSTATE[HY000]: General error: 2006 MySQL server has gone away: SELECT * FROM {menu_router} WHERE path IN (:ancestors_0, :ancestors_1, :ancestors_2) ORDER BY fit DESC LIMIT 0, 1; Array ( [:ancestors_0] => user/autocomplete [:ancestors_1] => user/% [:ancestors_2] => user ) ";s:9:"%function";s:15:"menu_get_item()";s:5:"%file";s:61:"/home/spjdco5/public_html/spjdc.net/drupal7/includes/menu.inc";s:5:"%line";i:465;s:14:"severity_level";i:3;} [:db_insert_placeholder_4] => 3 [:db_insert_placeholder_5] => [:db_insert_placeholder_6] => http://www.spjdc.net/node/add/simplenews [:db_insert_placeholder_7] => http://www.spjdc.net/ [:db_insert_placeholder_8] => 173.245.56.101 [:db_insert_placeholder_9] => 1369341274 ) in dblog_watchdog() (line 160 of /home/spjdco5/public_html/spjdc.net/drupal7/modules/dblog/dblog.module).
Uncaught exception thrown in shutdown function.

PDOException: SQLSTATE[HY000]: General error: 2006 MySQL server has gone away: DELETE FROM {semaphore} WHERE (value = :db_condition_placeholder_0) ; Array ( [:db_condition_placeholder_0] => 2061089831519e7d53bd2868.08307754 ) in lock_release_all() (line 269 of /home/spjdco5/public_html/spjdc.net/drupal7/includes/lock.inc).

Fatal error: Uncaught exception 'PDOException' with message 'SQLSTATE[HY000]: General error: 2006 MySQL server has gone away' in /home/spjdco5/public_html/spjdc.net/drupal7/includes/database/database.inc:2168 Stack trace: #0 /home/spjdco5/public_html/spjdc.net/drupal7/includes/database/database.inc(2168): PDOStatement->execute(Array) #1 /home/spjdco5/public_html/spjdc.net/drupal7/includes/database/database.inc(680): DatabaseStatementBase->execute(Array, Array) #2 /home/spjdco5/public_html/spjdc.net/drupal7/includes/database/database.inc(2347): DatabaseConnection->query('SELECT expire, ...', Array, Array) #3 /home/spjdco5/public_html/spjdc.net/drupal7/includes/lock.inc(167): db_query('SELECT expire, ...', Array) #4 /home/spjdco5/public_html/spjdc.net/drupal7/includes/lock.inc(146): lock_may_be_available('theme_registry:...') #5 /home/spjdco5/public_html/spjdc.net/drupal7/includes/theme.inc(449): lock_acquire('theme_registry:...') #6 /home/spjdco5/public_html/spjdc.net/drupal7/includes/bootstrap.inc(442): ThemeRegistry->set in /home/spjdco5/public_html/spjdc.net/drupal7/includes/database/database.inc on line 2168
Uncaught exception thrown in session handler.

PDOException: SQLSTATE[HY000]: General error: 2006 MySQL server has gone away in _drupal_session_write() (line 209 of /home/spjdco5/public_html/spjdc.net/drupal7/includes/session.inc).

Adding term reference and other fields to the content type does not appear to cause a problem. If I remove a term reference field I cannot add field_story07. IOW, it is not the total number of fields that causes a problem.