Castle <-> Reserve transportation
 

Here's the thing:

Troops you can put in a castle and reserve are unlimited. However, I have a castle all three slots of troops full and my main unit slots all filled plus my two reserves also filled, with all of these being different units. Now, the units in reserve and in the castle exceed my leadership. So the issue is that I want to switch a troop stack from my reserve straight into the castle garrison without having to first put them to one of my main slots (I can't do that because the troops exceed my leadership).
So I just want the mass of troops from the castle to my reserve while putting troops from the reserve to the castle.

Yeah -- this is a REALLY irritating bug, particularly when you're stocking up on particular units.