Fact-checking nightmares (prompted by spending a couple of hours looking up birth and death dates)

1.  Incorrectly noting somebody's date or place of birth and death.

2.  Bonus points for putting said somebody in the wrong country or century.

3.  Mixing up two authors with the same name.  (Not to be confused with the time I had to copy-edit a book review featuring both Sir Walter Raleighs.)

4.  Misspelling an author's name. (I did that at least once in Book One *sigh*)

5.  Incorrectly identifying an author's religion.  Needless to say, that's a potentially fatal error in my own line of work.   

6.  Incorrectly identifying the author of an anonymous work. 

7.  Assuming that a fictional work is factual, or vice-versa.  (I've seen the former happen at least twice.)

8.  Getting the date of a first edition wrong.  (This is a problem if your argument depends on the date of the first edition...)

9.  Assuming that an author was drawing on Now-Famous Book X, when Now-Famous Book X was not available or easily accessible at the time.

10.  Assuming that an author could not have known Now-Obscure Book Y, when Now-Obscure Book Y was, in fact, in the contemporary equivalent of Oprah's Book Club.