Oscillating Output from Arduino Pins when not in use, why? I have A1 set to be an input, have the program read it, and then print to the serial monitor. My question is, when the circuit is not receiving power, but is grounded, and is still connected to A1, when do I see an output pattern of, for example: 

0,0,0,0,7,23,16,9,4,0,0,0,0,3,18,21,15,0,0,0.....


I have a different detector inputing to A0 and if I don't have A1 connected then I get a reading that is similar to what I would see if I was actually reading A0 (around 900). Then if I plug back into A1 the value drops to around 350. When A0 is unplugged and only A1 is plugged in I only get noise (ie the pattern above in the code section).
I'm thinking along the same lines as CrossRoads and dandavis above. Would you be able to post a circuit diagram? If you haven't grounded the input, it could be floating or picking up noise from the surrounding environment. A 5k pulldown resistor may fix the problem, depending on what your circuit looks like and the environment its in.