[kennyguitar]

I don't know the rule number off the top of my head, but I had this discussion with my pro (I was in a lesson and we saw it happen, except the green was a practice putting green) and free drop no nearer the hole is the relief.

I'll see if I can find the Rule.

EDIT: Rule 25-3

25-3. Wrong Putting Green

a. Interference
Interference by a wrong putting green occurs when a ball is on the wrong putting green.
Interference to a player's stance or the area of his intended swing is not, of itself, interference under this Rule.

b. Relief
If a player's ball lies on a wrong putting green, he must not play the ball as it lies. He must take relief, without penalty, as follows:

The player must lift the ball and drop it within one club-length of and not nearer the hole than the nearest point of relief. The nearest point of relief must not be in a hazard or on a putting green. When dropping the ball within one club-length of the nearest point of relief, the ball must first strike a part of the course at a spot that avoids interference by the wrong putting green and is not in a hazard and not on a putting green. The ball may be cleaned when lifted under this Rule.

Penalty for Breach of Rule:
Match play - Loss of hole; Stroke play - Two strokes.