I have a programmable TV remote control ( a Marantz I've had since 1999 or longer if it matters), it uses 4 AA batteries. I often find (at least when I use Li-Ion, non-rechargeables) that when the batteries die, only one is bad, the other three are strong. They are installed in series. I measure the voltage on each when I install them, there is not a big difference in open circuit voltage (lowest of about 1.65 volts, highest is 1.73 volts). When the batteries die, it is about 1.66 volts for three, and some hundreds of millivolts for the one that is dead. This has happened often enough to make me wonder.
Is the power demand concentrated on one when it starts to fail? Why does just the one get drained?