First 'film-within-a-film'?
Charlie and Mabel visit a movie house and watch a short titled "Double Cross" (a Keystone Picture, natch).
Any history buffs out there know if this is the first example of a film-within-a-film?
No. Keystone did this several times in various shorts before Tillie. In around 1910 or so, Griffith had a film within a film in Those Awful Hats (which I think featured Mack Sennett).
shareGo figure that Griffith would do it first. I should've known.
shareGriffith didn't do it first -- others predated him.
British film pioneer R.W. Paul used the film-within-a-film device in a 1901 short called The Countryman and the Cinematograph. This was copied by the American pioneer Edwin S. Porter the following year, in Uncle Josh at the Moving Picture Show.
But that doesn't mean R.W. Paul was "first," either. With so many early films lost forever, film firsts in are difficult if not impossible to establish.