I was watching some other people's replays of AI Wave Survival games, and did observe in one, the Cybran T2 destroyers wouldn't path via a straight water route to the players. Looks like they were trying to path onto land (which actually was the optimal route to some players). I am guessing this was a very map specific issue, with some invisible barrier or map flaw preventing transition from water to land, as it hasn't happened on any of the maps I use.
I have had, on rare occasions, my ACU wander off half way across the map after being given large build queues. Seems it says, "Enough with this!" and decides to go for a walk. It is ok, I find it funny to hear the alert my ACU is taking damage, check my base where nothing is happening, and then a gigantic explosion half a map away!