When you believe characters like Robin Hood* not only really existed, but also met the Doctor.
*"Robot Of Sherwood" wasn't the first "Doctor Who" story in which a story we think of as a legend was revealed to be true, in the Whoniverse anyway. There are a few instances of this in the Classic Series:
1, The First Doctor not only got caught up in the Siege of Troy but also persuaded the Greeks to build the famous Wooden Horse.
2, At some point in his fourth incarnation, before he met Romana, the Doctor helped Theseus to defeat the Minotaur. This wasn't shown onscreen, but it was mentioned a couple of times.
3, The Seventh Doctor learned that a future version of himself was destined to become Merlin. Unless this has already happened offscreen, I think that story is long overdue for a sequel (or should that be prequel?), though it would be just like the show's writers to forget all about it.