This affects the current git tree only.
Sometimes when moving a unit, the adjacent fog squares don't clear. These fogged squares persist, and aren't cleared by your other units (but sometimes get cleared by indian units).
The behavior is intermittent. Out of 10 new games started, I note the misbehavior only about 4 times. So I can't give an exact set of steps to reproduce, but I've seen it on two different machines with separate git checkouts, and it doesn't occur with 0.10.7.
I've noticed it tends to occur more often when I exit a ship or settlement, however I've seen it occur sometimes when I haven't exited something. If I save and reload, the hidden square reappears, so that's effective as a work-around.
Possibly related to this, I've noticed Indians will sometimes reveal fog on squares I should actually not have visibility on. This seems to occur on squares near to my settlements.