RAll I know is, it was at 55%, the display timed out, I tried to turn the phone on a few minutes later and...nada. I kinda panicked, thinking the phone itself had died.
So I tried plugging it in, but it still wouldn't turn on (no charging light either). I took the back cover off, breaking a clip in the process (sigh), replaced my OEM battery, and I was good to go.
So I don't know what's up. Maybe a connection problem and the phone doesn't "see" the GTMax all of a sudden, since technically the phone won't run on AC if it doesn't detect a battery present, right?
Edit: I just did an experiment. I plugged in the phone and turned it off. The charge light stayed on. I pulled the battery and the charge light turned off. I put in the GTMax. No charge light. Replaced the OEM and once again the charge light came on. So clearly the phone isn't recognizing the GTMax for whatever reason. Maybe something got fried when I was playing Angry Birds, lol.