It's not infrequent for people to complain of broken RSS feeds and other behavioral oddities which are caused by extra whitespace in a custimized or extension PHP source file.
Usually this is a BOM character at the start or extra line breaks at the end inserted by a "helpful" text editor.
It should in many cases be possible to detect this condition, as well as other possible problems, by checking the value of headers_sent() somewhere in OutputPage, or perhaps even in Setup.php, before actual output is ready to start.
It should even be possible to scan all files included so far for the common problem conditions in this case, telling the user where to look and what to fix.
Version: 1.11.x
Severity: enhancement