The Notwithstanding Clause in the Canadian Charter of Rights and Freedoms allows for short-term circumvention of selected rights. This gives Parliament and provincial legislatures the power to overrule the court system and prevent protections under the Charter. This is a controversial clause and has never been used by the Federal Government, but the individual provinces have used it in certain situations.
As of 2012, no situation has existed in Canada that has seemed dangerous or threatening enough to invoke the clause nationally. Because it fundamentally removes rights from citizens -- inalienable rights being guaranteed in the Constitution -- it would be very unpopular and very risky for the incumbent Party. It could easily be seen as a naked power-grab or deliberate show of power over the citizenry; to justify it, there would have to be something intrinsically threatening the national security of the country as a whole.
As mentioned, however, the provinces have used the notwithstanding clause, or variations, for local issues; for example, Quebec used its own version to prohibit freedom of expression and equality in the case of public signs posted in languages other than French. This was a local issue and did not affect the other provinces.