TYPO3, character "" not allowed in prolog

This problem can also cause the error:
Error: no internal or external document type declaration subset; will parse without validation

Basically what is happening is the browser cannot read the document type and renders in quirks mode. This will often break your layout, in my case it completely broke the functionality of the site (ColorBox was borked).

The most likely solution? Convert any files with the encoding UTF-8 to UTF-8 without BOM. Some text editors automatically add the BOM, which is what happened in my case.