Manual talk:Hooks/OutputPageBeforeHTML

500 Server Error
When attempting to add text to $wgOut or parameter 1, an infinite loop is created. I guess that's why there's a disclaimer on the article page warning us to use at our own risk. But the second parameter is up for grabs. -- Egingell 12:10, 16 August 2007 (UTC)

It seems as if the first parameter is an OutputPage object rather than a ParserOutput object. (at least in version 1.6.3)

Simple Example
A simple example for using this hook is found in Extension:PatchOutput.