1. Can repair huts be used for anything? Using the repair feature doesn't seem to do anything, but it could be user error.
2. Repair huts cannot be destroyed, using the destroy tool just opens the repair hut dialog window.
3. Barracks dialog window can be clicked through to select different barracks (many dialog windows can be clicked through to the behind object). This happens frequently when you are recruiting and deploying soldiers.
4. When you have negative workers you cannot build anything (or trade anything, I think), including things that don't require workers (possibly by design, but it was really confusing until I figured it out).
5. Another example of UI being click-throughable is if your cursor is set to construct something, and you click in the build menu, you may build something on the tile behind where you clicked in the build menu.