I've has happened several times lately, but as an example, this morning my battery was at 99%. I was on facebook, and hit a link to a site - went to it through Firefox - and after a few seconds, the screen begins to flicker, and the phone shuts down. Upon restarting the battery is at 40%. Back of the phone got slightly warmer.
If this happens when I am under 70% to start, it pretty much drains the battery completely, and upon restart gives the 2 beep warning to plug in the charger.
(Samsung S2 - Sprint)
ONLY happens after going to internet, happens quite often.
And just to throw in a little conspiracy theory, it seems that this started happening after I started visiting some of the left and right wing politically charged websites. Has anyone had any similar experiences?