What is the consent exception to the warrant requirement?

The whole point of getting a warrant is that it is a necessary barrier and process to ensure that we only allow the government to violate people's privacy in certain, limited circumstances. Thus, if police receive consent to perform a search, there is obviously no need to still go out and get a warrant. After all, the ultimate permission slip is granted by the property owner. So this seems simple enough, right? On exams, it can get tricky based on other factors. Most commonly, permission is sometimes given out by someone who isn't the property owner, like a guest. For example, if Bob lets Sam spend the night at his apartment and, in the middle of the night, police knock on the door and ask Sam if they can come in, if Sam says, "Sure!" and the cops come in to find Bob's crack emporium, their discovery is fair game because Sam's consent is sufficient.

This works because Sam has "apparent authority" to give consent, since he opened the door. Practically speaking, any person who appears to be an adult can provide such permission to a visitor, like a police officer. Note, however, if Bob was standing in the room and heard Sam give the officers permission to enter, he could interrupt and say, "No, you're not allowed to come in." This would effectively revoke Sam's consent and require that officers first go get a warrant in order to proceed with their search.