I've been having weird battery issues with my slightly more than a year old Sprint Samsung Galaxy S2. Sometimes (maybe 4-5 times in the past 2 months) when I'm using a program it just seems to rapidly eat the battery and the phone dies, sometimes still claiming to have battery left and sometimes not, but if I reboot it claims no battery and quickly dies again. If I wait for a while though before turning it on, when it comes back I have more battery, and even if I don't plug it in the battery INCREASES for a while until it gets back to roughly what it should have been.
For instance, in the first picture I was playing a normally non-battery-intensive puzzle game when the phone suddenly died. During that brief outtage I did turn it on and it said 1% or 2% battery right before it died again. I waited a little while longer before turning it back on and the battery was up to about 30% now, and as you can see, it continued up without me charging it.
In the second case I was watching a video in the morning, which normally takes me down 10-15% of my total battery over 25-30 minutes, but this time killed all of the 80% remaining in 15 minutes (it died right after I took the screenshot, even though I was not playing video anymore). When I turned it back on later it was higher (though this time I did happen to charge it as well, and it charged to full off a keyboard plugged in usb cable in 30 minutes, which it would never do if it really was dead).
The only somewhat similar thread I've found was "solved" by buying a new battery, but I think that person had only just bought that battery so it was completely defective. One person suggested taking the battery out for 10 minutes, and I've done that but can't assess whether it did anything since this only happens every few weeks.