[_ck_]

First a quick tip: when you customise templates with a plugin function embeded, instead of this:

<? show_blah(); ?>

do this:

<? if (function_exists('show_blah')) { show_blah();} ?>

and that way if you deactivate a plugin, it will fail quietly and let the page still render.

Now to address your problem, that error can happen if something is trying to call bb_get_header before bbpress is completely loaded somehow. Or an include could accidentally get renamed. Or some whitespace got inserted at the end of a plugin somewhere (after the ending ?>)

You need more info. Go into your config.php and add this as the first line or so:

error_reporting(E_ALL);

And see what it throws at you.