PHP Output buffering contains something before script starts

i have a site, where i buffer some output with

ob_start();
... 

and it worked fine until today i updated my debian from an older php5.3 to the latest php5.3.3-7+squeeze8
Now i sometimes have something in the output buffer before i call it the first time

please don't answer things like

"header must be called before any output is sent."
(I know, I work a lot with output buffers)

when i set an extra ob_get_clean(); at the very first line of my script, it works

ob_get_clean();

it seems, like php is creating some output beforehand
if i put the first line

then i see, that there is an empty string already in the buffer:

""

on all other pages it isn't, there ob_get_clean(); contains

null

Answer

i found it:

i had no invisible character in front, it was something different: i called ob_end_clean() one time too much:

this was my code, inside a function i call:

function print_something(){
ob_start();

echo some stuff...

echo ob_get_clean();
ob_end_clean(); // this was the bug! 
}

it seems, that you can clear your main output buffer ;)