Webkit crashes with
Unhandled exception at 0x0245ae87 (Qt5WebKitd.dll) in BugWebkitAnimation.exe: 0xC0000005: Access violation writing location 0xbbadbeef.
in
C:\Qt\Qt5.2.0\5.2.0\Src\qtwebkit\Source\WTF\wtf\Assertions.cpp: WTFCrash()
when using CSS3 animations.

I could reproduce it when using a <div> which has a custom fade-in animation contains a <div> which also has this fade-in animation and a <div> without that animation:
<div class="fade-in">
<div>
Non-fading
</div>
<div class="fade-in">
Fading in
</div>
</div>

See the attached HTML file and a very basic CPP program to load that HTML file.
The crash occurs only if the two "fade-in" animations start at the same time as in the provided files.