A Contact with a Pending membership cannot be merged with another Contact due to missing membership End Date. The membership End Date is not set on a pending membership.

The sequence of events is:

1. Contact signs up for new membership
2. Contact already exists in CiviCRM (as a non-member) - or is similar contact
3. CiviCRM Admin tries to merge the two Contacts
4. The generic: DB Error: unknown error is then raised

Copy of error from CiviCRM logs.

Dec 06 12:32:20  [info] $Fatal Error Details = Array
(
    [callback] => Array
        (
            [0] => CRM_Core_Error
            [1] => handle
        )

    [code] => -1
    [message] => DB Error: unknown error
    [mode] => 16
    [debug_info] => UPDATE civicrm_membership SET end_date = '' WHERE id=281 [nativecode=1292 ** Incorrect date value: '' for column 'end_date' at row 1]
    [type] => DB_Error
    [user_info] => UPDATE civicrm_membership SET end_date = '' WHERE id=281 [nativecode=1292 ** Incorrect date value: '' for column 'end_date' at row 1]
    [to_string] => [db_error: message="DB Error: unknown error" code=-1 mode=callback callback=CRM_Core_Error::handle prefix="" info="UPDATE civicrm_membership SET end_date = '' WHERE id=281 [nativecode=12
92 ** Incorrect date value: '' for column 'end_date' at row 1]"]
)

Agileware Ref: CIVICRM-1122