Barbara Schmidt may have a point here about Twain's attendance at
Whitman's 1887 Lincoln lecture in Madison Square Theater in NYC.  I
stated in my biography (WALT WHITMAN: THE SONG OF HIMSELF, 1999) that
Twain was there. My source was William Sloane Kennedy's REMINISCENCES
OF WALT WHITMAN, 1896 (a volume I can't right now put my hands on).
I too found no journalistic references to Twain's presence that
evening in the NEW YORK TIMES and probably should have thought twice
about including that information.  Other early sources for the
lecture which cite accounts of attending celebrities and exclude
Twain are William E. Barton's LINCOLN AND WHITMAN, 1928, and Clara
Barrus's  WHITMAN AND BURROUGHS, COMRADES, 1931.  Justin Kaplan does
not put Twain at the lecture in either his Twain or Whitman
biography. Daniel Mark Epstein's recent study of Whitman and Lincoln,
2004, includes Twain at the lecture but, as Ed Folsom notes, gives no
clear or originally cited source.

Gay Wilson Allen's biography of Whitman, THE SOLITARY SINGER, 1955,
also states that Twain was present, but it gives no source for the
information.

Now that I am working closely with Twain, it strikes me as odd that
no contemporary account would mention Twain, who was probably the
most famous of the group in question.