From the specification:
4.8.10.16 Best practices for authors using media elements
Playing audio and video resources on small devices such as set-top boxes or mobile phones is often constrained by limited hardware resources in the device. For example, a device might only support three simultaneous videos. For this reason, it is a good practice to release resources held by media elements when they are done playing, either by being very careful about removing all references to the element and allowing it to be garbage collected, or, even better, by removing the element's src attribute and any source element descendants, and invoking the element's load() method.
Similarly, when the playback rate is not exactly 1.0, hardware, software, or format limitations can cause video frames to be dropped and audio to be choppy or muted.
source: http://dev.w3.org/html5/spec-author-view/video.html#media-resource