How to debug: "HEADER ALREADY SENT" and GD2?

Question

I have an annoying HEADER ALREADY SENT error.

It is not easy to debug it, what I could do was to improve the logging to isolate the source of the issue (has described here):

Cannot send headers; headers already sent in lib/Varien/Image/Adapter/Gd2.php, line 133

Here line 133 of Gd2.php:

call_user_func($this->_getCallback('output'), $this->_imageHandler);

And it is part of the method display:

public function display()
    {
        header("Content-type: ".$this->getMimeType());
        call_user_func($this->_getCallback('output'), $this->_imageHandler);
    }

How to go on ?

Answer 1

Try this

Disable compilation and refresh cache