I once built a high input impedance amplifier and used two
1N914/1N4148 (or similar diode with transparent case) to limit the input voltage range between Gnd and Vcc. I noticed sudden voltage fluctuations when working with the circuit and suspect some oscillation problems.It took a while, before I realised that the fluctuation occurred, when my finger was close to one of the diode, shading the light from the lamp. The illuminated diode generated a larger current, which caused the fluctuation.
Putting black tape on the diodes solved the problem. The moral of the story is to use non-transparent cases in circuits with high impedance levels.
Paul