Effects of no-clean flux or RMA flux residue on signals
I have run into this problem with precision photodetection circuitry. After reworking a board any leftover flux threw off the instrument calibration due to increased surface leakage bypassing guard features on the PCB. This was on the order of 500pA to a few nA depending on how thick the flux layer was. Cleaning the amplifiers very carefully with IPA and a small amount of acetone as a drying agent fixed 'er right up. Generally I do not worry about no-clean flux residues outside of precision amplifiers, potentially out of my own ignorance. I have yet to attribute an observed problem with no-clean flux in non-precision or non-critical circuits. Case by case like most things I suppose.