I ordered a pioneer in my city to move to a very distant resource on the map.  A few turns later, I noted it was near a mountain range.  Quite a few turns later I discovered that the pioneer was just sitting at that spot, not moving at all.  It seems to have "given up" on getting to the location and just stopped moving.

This isn't good: 

• It wasn't *that* tricky a path to get from the city to the sport I clicked on - another pioneer got from the same city to a slightly closer spot.
• If the unit "can't get there from here", it should tell me that *immediately*; not just move for a few turns and then stop.
• If a unit does need to stop for some reason, I should get a warning message of some sort.