So on Sunday night at the Oscars, Skyfall and Zero Dark Thirty tied for Sound Editing. While it is rare, it is not unprecedented: Probably most famously Katharine Hepburn and Barbra Streisand tied for Best Actress at the 1969 Oscars when they won respectively for A Lion in Winter and Funny Girl.
But there were other ties! Brian Stelter pointed out a database showing that there were also ties in 1949 for Documentary (Short Subject), 1986 for Documentary (Feature), and 1994 for Short Film (Live Action). In 1931/2 there was a sort of tie in which Frederic March and Wallace Beery technically tied, though it is noted: "Not a true tie, as Fredric March had one more vote than Wallace Beery, but rules at the time stated that ifan achievement came within three votes of the winner, that achievement would also receive the award. The rules have since been changed, and only an exact match in totals would qualify as a tie." Classicmoviechat.com describes that incident more fully.