Straight from the R&A website:
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.
Q. And finally, double-greens. Does Rule 25-3 apply to a double-green?
A double-green is one putting green that is shared by two holes: the second and 16th holes on the Old Course are a perfect example. If, while playing the 16th, a ball comes to rest on a part of the green close to the flagstick for the second hole, as this is one putting green, Rule 25-3 does not apply. The player must play the ball as it lies, even if it requires a putt of 50 yards or more!