Let's say the police show up at a home you share with several people and have a search warrant. If they are trying to find evidence of drugs, they can look through most of the home and, if your housemates have any contraband in the home, there could be serious consequences. But those consequences may not be restricted solely to your housemates. During the search, if they find drugs in the common areas of the home, they may believe you are culpable, as well.
Fortunately, that is rarely enough to convict someone of a drug crime in Ontario. The Crown would likely need more evidence linking an individual to the drugs before they could hope to convict. Unfortunately, however, some people think even this small amount of evidence would be too hard to fight, and say things that could potentially incriminate themselves.