When a post having a custom bbcode which contains as example a smile in the first position it's shows correctly.
But if the post need to be edited, the trim ( in the first_pass_replace row at phpbb_bbcodes table ) makes the smile not working, because in phpbb3 "Smilies always need to be separated by spaces." ( root/docs/README.html at 6. Overview of current bug list )

'[center:$uid]'.str_replace(array("\r\n", '\"', '\'', '(', ')'), array("\n", '"', ''', '(', ')'), trim('${1}')).'[/center:$uid]'