Hypothetically Speaking

[natgolfer]

In this imaginary, but possible situation, A and B are partners in a "SELECT DRIVE / ALTERNATE SHOT," format. Both partners drive and then A, mindlessly, hits his own ball. After realizing the mistake A then hits B's ball on to the green after which B makes the putt for a 3.

Do the rules of golf cover a situation like this?

Would the team be penalized for A taking a "practice" stroke?

[Kilroy]

My guess would be A played a wrong ball.
He must abandon his ball and play b's ball as you describe, but...
In stroke play - 2 stroke penalty - score is 5
In match play - loss of hole.

But why would they be obliged to play b's ball? It seems to me that after A hit his ball, they should have picked up B's ball, having already chosen A's to play (sort of). Had they done that, the penalty could have been avoided.

[Gary Hill]

In foursome play (alternate shot - two playing against two and each side playing one ball):

If the partners play a stroke or strokes in incorrect, the stroke or strokes are cancelled.

The side is penalized two strokes and the error must be corrected by playing a ball in the correct order as nearly as possible at the spot from which it first played in incorrect order.

If the side plays a stroke from the next teeing ground without first correcting the the error, the side shall be disqualified.

In four-ball stroke play (two competitors playing as partners, each playing his own ball):

If a competitor plays a stroke or strokes with a wrong ball except in a hazard, he shall add two penalty strokes to his score for the hole and shall then play the correct ball. His partner incurs no penalty even if the wrong ball belongs to him.
If the wrong ball belongs to another competitor, its owner shall place a ball on the spot from which the wrong ball was first played.

In select drive/alternate shot, you can do whatever you want because it isn't actually golf.