Many landlords insist on adding certain clauses to rental lease agreements, however in many jurisdictions, these clauses are deemed void.
For examples, in Ontario the most commonly reported void clauses in rental lease agreements are:
No Pets Allowed
Requiring damage deposit
Requiring content insurance
No overnight guests
Therefore, if you are signing a rental lease agreement in Ontario, and the landlord ads any of the above clauses, these clauses are void and hence cannot be enforced.