If a line in a code block is indented by a single space, then it comes out as not being indented in the final post:

0 indent

 1 indent

  2 indent

   3 indent

    4 indent

Hmm, actually, it seems to be working here on the bug tracker, since the CSS here has white-space:pre ... try it on an actual phpBB forum, and it will fail - the "1 indent" line will not be indented, though all the others will be indented correctly.

The fix is very simple, just a couple of lines added to bbcode.php - a diff -u for the current trunk has been attached.