Since there are no first-hand accounts of Shakespeare's personality (unlike Samuel Johnson, he had no sycophantic Boswell recording his every move) early biographers had to rely on hand-me-down anecdotes from ancestors of next-door-neighbors of drinking buddies, etc. Filtering out all the inevitable augmentation and exaggeration as the stories pass from one ear to the next, many of these legends could conceivably have a kernel of truth in their origin.
My favorite, hands down, comes from a book called "Shakespeare's Jests, or the Jubilee Jester," published around 1769 in the flurry of memorabilia following Garrick's Stratford Jubilee, dealing with the friendship/rivalry between Shakespeare and Ben Jonson:
"Shakespear seeing Ben Jonson in a neccesary-house [on the toilet], with a book in his hand reading it very attentively, said he was sorry his memory was so bad, that he could not shite without a book."
Playwright Peter Barnes, in the intro to volume 2 of his anthology, calls it "the only good joke Shakespeare ever made... if he made it." Barnes, as you can probably guess, preferred Jonson.