"I think I just failed a Spot check."
- Belkar Bitterleaf, "Order of the Stick", by Rich Burlew
This article in Slate discusses a Supreme Court case in which a police officer was summoned to a domestic disturbance and when he got there, the husband was not home. The wife claimed her husband was dealing drugs and allowed the cop to search the house. During the search the husband got home and demanded that the search stop. The cop did not stop the search and eventually found drugs. The question of course is whether or not it was a legal search because the wife agreed to it, even if the husband did not.
One hypothetical situation that was not discussed in the article was: what would have happened if the husband did not show up before drugs were found, and only later claimed that the search was illegal because the wife did not have the authority to consent for him? If this would be considered illegal, then if I were a drug dealer, I would make sure to have an accomplice waiting in the wings to swoop down and claim the drugs were his when the police found them in a "consented" search, thus making the search illegal. The police would have no way to know about this plan in advance (even if they asked me, I could tell them that all the stuff in my house was mine, and the accomplice could argue that my false statement does not mean that the violation of his rights was legal). Therefore, the only way for the police to protect themselves against this tactic would be to wait for a warrant (if they had probable cause) even if the subject consents. This would significantly inconvenience innocent subjects, because they would have to be detained until a warrant was issued - even if they had nothing to hide and were perfectly willing to let them search right then and there!
If you accept the argument of the husband in this case that the wife did not have authority to consent for him, then presumably this fact would not depend on whether or not he happened to return in the middle of the search, so you would end up with the conclusion above.