I have rather complicated architechture in my php-program (including a kind of programhandler...)
In one php-file I tried to send an pdf-formatted file to the browser by using "readfile" -function. For my huge appointment it did not work well with "if" and "case" clauses. And the most imazing was that exactly same script worked in an other php-file (where the architecture was simplier). 
I give you a simple example how the "bug" behaves
*********
$i=1;
if ($i==1)
  readfile(example.txt);  //and nothing happens
*********
$i=1;
if ($i==1)
  readfile(example.txt);  //this worked well
else
  readfile(example.txt);  //this was just for debugging
*********

As you see, it seems to be no sence at all!!! 
You propably think that I had somethin wrong with my code, but no, I and my work mate debugged this problem a half day and I am sure that this bug was just like I said.
I thought that I could have somethin wrong with the libraries but no error occurs...
The libraries are self made and working well in other programs.