I'm using an analog input of an Arduino UNO to monitor battery voltage for a robot. The battery is a 2-cell Li-Ion stack producing about 7.2 V.
Rather than use a voltage divider on the total battery stack to get the monitored voltage within the allowable analog input voltage range, I decided to simply monitor the voltage of the bottom cell in the stack. Unfortunately, this means that when the battery is disconnected from the robot and Arduino via the main power switch, the 3.6V from the bottom cell is still present at the Arduino input pin.
The 3.6V at the analog input pin is clearly back-flowing into the Arduino to some degree, as the power LED illuminates, and the loaded program appears to run. Is this a feature, or a bug, or what? Although I haven't seen any evidence to date that this back-flow is damaging my Arduino, it is at least very worrisome.