Suppose you have just made a nice amplifier and are testing it out with a sine-wave input. You switch the input function generator to a square wave, but the output remains a sine wave! You don't have an amplifier; you've got trouble. Parasitic oscillations aren't normally as blatant as this. They are normally observed as fuzziness on part of a waveform, erratic current-source operation, unexplained op-amp offsets, or circuits that behave normally with the oscilloscope probe applied, but go wild when the scope isn't looking. These are bizarre manifestations of untamed high-frequency parasitic oscillations caused by unintended Hartley or Colpitts oscillators employing lead inductance and interelectrode capacitances.