This is absolutely true. Many people do not realize when they immigrate what a common law relationship is because their country may not have anything like that. Therefore, they may list their status as single in good faith. At some point after PR they get married and when they try to sponsor their spouse and show in their relationship history that they lived together as a couple for 12 months before the sponsor got their PR, they get a bad surprise. The sponsor is then guilty of misrepresentation and may lose their PR and even if they get to keep it, they can never sponsor that spouse.computergeek said:Leon's answer was absolutely correct, but I want to quash one mistake I've seen people make.
If you get married and then land in Canada without declaring your spouse, you will not be allowed to sponsor your spouse, ever. This also applies even if you have just been living together for one year or more prior to landing - this is known as a "common law relationship" and is equal to being married in the eyes of CIC.
If one marries or the time of a common law partnership clicks over to 12 months before landing as a PR, the spouse must be added to the application before landing. This is very important.
Of course if the person is an adult dependent child, it would however not be advisable to get married or enter into a common law relationship before landing because that can affect their status as a dependent child and exclude them from the parents application.