The function chat_prepare_update_events in mod/chat/lib.php (MOODLE_34_STABLE) makes an incorrect assumption that causes an exception in the cron.

Line 497:

$DB->get_field('event', 'id', array('modulename' => 'chat', 'instance' => $chat->id))

This assumes that there will never be 2 separate entries in the event table with the same instance and modulename. This is incorrect. Chat creates a 'chattime' entry for a scheduled chat, and a 'expectcompletionon' entry for expected completion date.

When these both exist, get_field throws an exception.

The comment for the function clearly anticipates this, but the code doesn't.