Untapping an untapped minion triggers?

You're mixing up two things:
- things that does nothing (eg. tapping a tapped vampire, stealing blood from a vampire with no blood)
- things that are forbidden by the rules.

Tapping a tapped card is not forbidden (unless the effect asks to "tap an untapped X"); however, it does nothing.

In details:

2wayspeaker wrote:

Either you stick to the definition of tap from the rulebook and you cannot tap a tapped minion
If Tap and Untap are defined as follows:

Ankha wrote:

Tap: Turn (a card) sideways. Typically done to indicate that the card has been activated for some purpose.
Untap: Restore a card to the normal, upright, position. See Tap.

If the card doesn't turn (because it's already turned sideways or upright), then it doesn't tap or untap per definition. It *stays* tapped or untapped.

Then you cannot tap a tapped minion and you cannot untap an untapped minion. There is no action to be performed here. (You cannot close a closed door, you can only open it and then close it again as per the definition) Because the definition includes that the card has to change orientation.

A tapped card can't be tapped (mecanically; as you said, you can't close a closed door). However, you can choose to tap a tapped card: nothing happens.

2wayspeaker wrote:

In order to tap you have to turn from upright to sideways. Anything less than this does not constitute tapping according to the definition.
In order to untap you have to turn from sideways to upright. Anything less than this does not constitute untapping according to the definition.

Correct.

2wayspeaker wrote:

Therefore you should not be able to tap a tapped minion if you cannot change the orientation there is no tapping to be performed.

If the effect doesn't state that the target must be an untapped minion, you can play a "tap" effect on it. That part will do nothing, though.
It's the same as stealing blood from a vampire that doesn't have blood.

2wayspeaker wrote:

Impossible, you can't do it and shouldn't be able to do it. This is similar to trying to resque a ready vampire from torpor.

It's not similar, as you can rescue from torpor only vampires in torpor, per rules (6.5.3. Rescue a Vampire from Torpor)

2wayspeaker wrote:

Or you agree with the ruling that you can tap a tapped minion
<snip>

The rest is moot.

Here's a list of other things that are legal to do but have no effect (or a partial effect):
- dealing damage to a minion that is immune to damage (Talbot Chainsaw / Ponticulus, or Abyssal Hunter / Carolina Vasquéz)
- playing a Villein on a vampire with 0 blood
- playing a Villein on a vampire with 1 blood (drains as much as possible, that is 1 blood)
- playing a frenzy card against Gotsdam, the Tired Warrior (but not against Aleister Crowley)