- Select a unit [let's call it "unit #1"] and target a distant stack of units [let's call it "stack #2"] as the destination;
- It's a few moves away, so use rail movement;
- Meanwhile, select unit(s) [let's call them "unit(s) #3"] in the destination stack ["stack #2"] and move them somewhere else (leaving one or more units behind, since it's the destination target for the above unit ["unit #1"]);
- The original moving unit ["unit #1"] now targets the new unit/stack ["unit(s) #3"] destination, forgetting the original one ["stack #2"] (I've also seen it just stop moving).
This can have really bad consequences, such as the original unit getting stuck in the middle of a lake/ocean region, running out of supply, and disappearing.

I also notice that a new unit/stack taken from an existing one "inherits" the rule of engagement (RoE) posture.
Looks like the destination target status also transfers.