Adding insulation over a wet waterproof layer for stops or when it's colder at altitude can be a sensible clothing strategy, especially if taking off your waterproof shell to add mid-layer insulation is going to result in all your layers becoming saturated with rain.
Water trapped between your Shell and Over layer will begin to evaporate in the warmth and will diffuse through your layers to regions that are less humid. The moisture can move outwards and inwards!
This "reverse breathability" can cause your skin and base layers to become damper before the rain water is driven out by body warmth as the temperature/humidity gradient normalises. It is most noticeable when the Overshell is warmer than your skin temperature. To reduce these problems remove as much of the rain water as possible before adding your over layer and put it on before you become chilled. Changing layers inside a ShellTA can help as its hydrophilic fabric absorbs some of the rain, acting like a giant towel.
Reverse breathability can also be noticed if you wear wet FurTech garments inside a car with the heater going, or in a warm pub or mountain hut. However, I often put up with this, as they dry quicker when I wear them.