[Mobhack]

You cannot undo if there has been something that changed - e.g. somebody spots the unit, or shoots at it. Also the undo attempt must be made immediately - if you say turn the units turret, then no undo. Probably if you selected another unit and then went back and hit undo.

Also in PBEM - undo is disallowed to stop "recce by undo" - move a unit, press undo, if it undoes then no enemy have spotted it, rinse and repeat till undo does not do so = unspotted enemy must have LOS at that point.

Nb - if the enemy infantry tried and aborted an assault on it, then that is an "action" and so an undo is no longer possible even though it was not reported. Aborted assaults are not reported, other than ones where they decide to shoot instead.