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The issue is closed by apple due to duplication:
Apple Developer Relations20-Nov-2014 08:27 AM

Engineering has determined that your bug report is a duplicate of another issue and will be closed. 

The open or closed status of the original bug report your issue was duplicated to appears in the yellow "Duplicate of XXXXXXXX" section of the bug reporter user interface. This section appears near the top of the right column's bug detail view just under the bug number, title, state, product and rank.

If you have any questions or concerns, please update your report directly here: http://bugreport.apple.com/.
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Summary:
In some cases, when trying to hide the captions using JavaScript (Setting the mode of texttrack within video tag to "hidden") using iPhone's Mobile Safari, the browser crashes. This happens on iOS simulator, iPhone, iPod and iPad (only in fullscreen video playback) with iOS 7.

Steps to Reproduce:
Open the page testtrack.html in CrashPage.zip in iOS, the player in the page will first load a video, and add a caption. Then after 2 second it will try to change the source of the video. After the video source is changed, the player will try to hide the caption, resulting in crash of the browser.

Expected Results:
Browser should not crash.

Actual Results:
Browser crashes instantly when try to set the caption mode

Notes:
iOS 8 devices does not crash in this case, but the caption in previous video will remain on the screen, and only closing and then replay could remove the caption.

Configuration:
All devices and simulator (when playing in fullscreen).