Today I wanted to be super safe and check all connections on my circuit if there are some shorts. To my horror GND and +5V seemed to be connected - and I couldn't figure what I did wrong.
I removed the Arduino (Nano) from the circuit - and the short was gone!
Then I checked a bare Arduino Nano (and also a Uno). Same result - connected (means: resistance between pin GND and pin +5V is only about 1200 Ohm).