Yasser Seirawan tells a good story here about Bronstein's tribute game with Walter Browne. But he doesn't say which exact move caused Walter to think for an hour. There's a discussion on Chessgames about what the move might have been, but nothing definitive as far as I can tell.
I guess we could speculate some more, but I'm really wondering if anyone out there knows for sure. Maybe we could e-mail Yasser or Walter? 😉