The biggest problem in games is the number of units - you might send 12 units through a narrow pass, and the game needs to have the units go through smoothly without blocking each other too much. From what I remember, the shipped version of Starcraft still had problems where you'd send a group somewhere, they'd get a little bit stuck going through, and one or two units at the back would then re-path to a really convoluted round-trip rather than just waiting half a second then continuing their original path.
The bigger units like dragoons and goliaths were notorious for stupid behavior in groups.. so it was never really resolved, and became a feature that progamers had to deal with.
