Factual error: When Marylin Rexroth's ex-husband dies, she inherits his money because he had not removed her from his will after divorcing her. Under California Probate Code § 6122(a)(1), the dissolution of a marriage automatically revokes any disposition or appointment of property made by the will to the former spouse.
Continuity mistake: As soon as George Clooney tears up his speech, we see it scattered on the podium. As he leaves the podium, we see his torn up speech placed neatly to one side, but the pieces are quarter-sized, despite only tearing them once. He walks down the aisle, then in the next shot from the podium watching him walk away, the papers are scattered again.
Audio problem: The producer sings along Simon and Garfunkel's "The boxer", but changes the lines "Just a come on from the w*ores on 7th. avenue" to "just a come on from those gals on 7th avenue". I'm sure the Coen Bros. did it deliberately. And strangely enough, when the song is played during the final credits it's the original text.