[phpBB Debug] PHP Warning: in file [ROOT]/posting.php on line 1216: implode() [function.implode]: Invalid arguments passed
[phpBB Debug] PHP Warning: in file [ROOT]/posting.php on line 1307: implode() [function.implode]: Invalid arguments passed
[phpBB Debug] PHP Warning: in file [ROOT]/includes/functions.php on line 4660: Cannot modify header information - headers already sent by (output started at [ROOT]/includes/functions.php:3815)
[phpBB Debug] PHP Warning: in file [ROOT]/includes/functions.php on line 4689: Cannot modify header information - headers already sent by (output started at [ROOT]/includes/functions.php:3815)
[phpBB Debug] PHP Warning: in file [ROOT]/includes/functions.php on line 4690: Cannot modify header information - headers already sent by (output started at [ROOT]/includes/functions.php:3815)
[phpBB Debug] PHP Warning: in file [ROOT]/includes/functions.php on line 4691: Cannot modify header information - headers already sent by (output started at [ROOT]/includes/functions.php:3815)

Caused by clicking Preview when I was editing a post. I tried to reproduce it with a few random other posts and it happened 3 out of 5 times.