Oh, I forgot to mention that when you probe a circuit, the presence of the probe can affect the operation of the circuit.

On RF designs it was not uncommon to find seemingly needless small valued components at certain probe points. The idea was these modeled the reactive load of a probe. When you needed to probe it you'd remove the components and attach your probe and the circuit see approximately the same load so you could have more faith that what you were seeing more closely matched what was happening when you weren't looking.