Two known unit stack bugs. First bug. 1. Group two colonists. 2. Send them to an indian village for training. 3. With the units still grouped click on the book to begin traning. 4. After traning is done the units are immobile. Second bug. 1. Group two ships. 2. Click on the icon to sail for Europe. 3 Watch the ships get stuck before or after the trip to Europe. Description When you issue special orders to grouped units the bug causes them to get stuck on the tile. Workaround Don´t issue special orders to grouped units.