I've observed some rather serious issue in QT 7 & X yesterday which must be caused by a buggy h.264 component.
QT renders the colors of my latest h.264 files way too bright, while VLC and the Adobe CC decode them correctly. I've already observed that issue happening across different web browsers and video players back in 2013, which has led me to write an article about it back then; But by the time I had documented the whole shabang and was about to release the article, the issue had magically disappeared - there must have been a system or software update that fixed the issue.
Now the issue is back.
A client of mine has been repeatedly complaining about the luminance of a color, which I already had lowered a few times. And as it came out, he's on the latest Mac Pro using the Quicktime Player. I told him to check the file in VLC, and he confirmed that everything was fine.
I'll be telling my clients to use the VLC player in the future, and so should you.
I'm on OSX. 10.8, by the way.