How to prevent SOIC8 clip from backfeeding
If you want to avoid strange problems from the microcontroller, hold its RESET pin on (usually low) and it should halt it from doing anything. For AVRs (Arduino's usual microcontroller) it resets internal registers to defaults, which means that everything is an input.

Other microcontrollers or components may do strange things, however. Without looking at a circuit diagram there's not much telling.