If a landmark town center/barracks/whatever building is destroyed, then you are unable to produce units from the surviving buildings of the same type.
- Build a 2nd town center
- Lose landmark town center
- Use select all town centers hotkey
- Be unable to queue villagers from the surviving town center
I suspect this happens for other landmarks that are grouped together with other building types. However, this MIGHT only happen if the landmark is the 1st building built of that type.