For example, suppose you were intending to rebid NT but an opponent overcalls in a suit in which you have no stop. Similarly, if your second suit just happens to be the enemy suit you may be stuck for a rebid.
In these situations, if partner forces you to bid by doing a Sputnik double or forcing suit switch you may have to be a little economical with the truth. Your options are:
The Sputnik doubler needs to be sympathetic to these possible difficulties faced by opener. If opener bids NT opposite your Sputnik double, don't assume he has a stop in the enemy suit. He may also have a less than ideal NT shape and be relying on you to stop both the unbid suits. Unless your own hand is genuinely suitable for playing in NT, you should probably take it out.
If opener rebids his suit opposite your Sputnik double you can assume he hasn't got much in your suits but don't assume that his suit is, necessarily, all that wonderful. He may have a holding in the enemy suit but a hand too weak (or the wrong shape) for a NT rebid.