Couple of questions...

[Mobhack]

1) as above - if you get spotted, spot a new enemy, or get shot at, Or an enemy unit assaulted you then you cannot undo a move.

Also, not allowed in a PBEM game. That stops "recce by move then undo umpteen times" - we let you do that versus the AI if you want to, its up to your own conscience.

Also - if you turned the unit (or its turret) at the terminal hex then that cancels the undo for any movement it just did since a turn to face is a new spotting event i.e. a search for enemies. (turns the thing made in the movement to folow roads etc. dont count, its if you yourself try to face a new direction at the end of the movement that cancels move undo as the turn to face.spot is a new movement event of itself)

Also - IIRC, an "All" group move cannot be undone either (using A key).

The undo really has to be done immediately as an OOPsie! to be effective. If you do other stuff and return to the unit, you probably wont be able to undo.

2) Units can leave rooster trails of smoke.dust in certain conditions of high visibility especially on desert maps. Sometimes it is just movement on earth roads in green terrain.
IIRC, that is another thing that cancels undo move since you cannot undo if you made new smoke puff in a hex.
The rooster trail generation will undo the clear smoke action since any new smoke or fire flips the flag back on.