When: June 2007
After his relationship with Jessica Simpson ran its course, John was briefly rumored to be dating another pop star-turned actress, the gorgeous Mandy Moore. Before she got hitched to crooner Ryan Adams, Ms. Moore had been linked to her fair share of famous beaus, so if this hook-up turned out to be true, it wouldn't shock us in the least. However, once the news broke, Moore's reps quickly dispelled this relationship as nothing more than friendship.
Smash Probability: Let's be honest here, no PR firm is putting out a press release saying "Yes, my client is boning John Mayer," so her reps' statement means nothing to us. But, since the claim has been outright denied, we'll give Mandy the benefit of the doubt.