The Java FX 2.1 Media stack on Mac OS X currently uses the Video Decode Acceleration (VDA) framework (https://developer.apple.com/library/mac/#technotes/tn2267/_index.html) to decode H.264 frames. This approach has several limitation:

1) The framework only supports certain graphics cards and not all Mac models are equipped with such compatible GPUs.
2) Even if a Mac is equipped with a compatible GPU, only one instance of the VDA decoder may be instantiated at one time, so, if another process has already allocated the GPU, H.264 decoding will fail.
3) Even if a Mac is equipped with a compatible GPU and Java FX Media is able to allocate it, H.264 decoding might still fail if the hardware decoder cannot decode the specific video track attempting to be played or it does not support the rendering format required.

As a result of the foregoing, at most one H.264 video may currently be played on a given Mac.