Supposedly, half-way through a screening of Rivette's Celine and Julie Go Boating (1974), Pauline Kael got up, announced "I'm going to the movies," and left.

Supposedly, half-way through a screening of Bergman's Autumn Sonata (1978), Alfred Hitchcock got up, announced "I'm going to the movies," and left.

If I had to guess, I'd say the Kael incident really happened and was later misattributed to Hitchcock, but there's no way to know for sure.