Measuring voltage causing power supply sourcing issue

I am measuring/monitoring the voltage of the 5 volt supply using the A6 pin of a Nano. Everything worked fine until I needed to occasionally toggle the power of the nano. Now when I remove the 5volts on the VIn line the nano stays powered !!!! The only way I can shut off the nano is removing the 5 volts from the A6 pin. Should this happen ? If so how can I prevent voltages on analog inputs from powering the nano ?

Do not apply an external voltage which exceeds the supply voltage to any pin directly - this will back-power the board through the protection diodes, which will exceed the rated maximum current through the protection diodes and potentially damage the arduino. This is how you blow pins.

If you can't arrange for the voltage to not be present on the pin when the board is not powered, put something like a 10k resistor between the analog pin and the external voltage; this will keep the current low enough to prevent damage.

If you put 5v onto the Vin pin, you'll get 3.something on the 5v pin - Vin goes through the regulator, which has a minimum dropout of 1.something volts. If supplying with external 5v, you need to put that on the 5v pin, not the Vin pin. If supplying via Vin pin, you need to give it at least 7v to get 5v out.

Thank You for your answer. So to be clear, the power input (VIn) voltage should always be above what to be expected on the data I/O pins , correct ?