When viewing the [code] bbcode and then clicking the select all button to select all, theres always a preceding whitespace before the code when pasting it to any notepad. This confuses many newbies when copy pasting code, and they come back with variations of this error:
[phpBB Debug] PHP Notice: in file /includes/session.php on line 916: Cannot modify header information - headers already sent by (output started at **removed**/test.php:1)
This whitespace problem has been fixed by a mod, however I cannot remember the location of the mod (it is on phpbb). This is a simple problem but causes a large amount of questions from newbies about why this error exists, which can easily be negated by removing the whitespace.