Today I listened to Jascha Horenstein's 1959 live recording of the Mahler Eighth, one of the most electrifying documents of a performance in existence. This being Royal Albert Hall, the audience is primed to explode, and right after the last blast of E-flat you hear what sounds like a simultaneous "Bravo!" from two or three people in different parts of the hall. The rest of the crowd starts screaming a split second later. I then put on a 1988 Proms recording of the Busoni Piano Concerto, with Mark Elder conducting the BBC Symphony and Peter Donohoe doing a hair-raisingly brilliant solo turn. Here a solitary soul manages to get in his "Bravo!" a beat before everyone else, perfectly punctuating the demented giddiness of the ending. At the Met and elsewhere, overzealous bravo artists can sometimes have a ruinous effect, but in Royal Albert Hall the diehards know their cues. I can't imagine these two favorite recordings without the anonymous yelps of joy; they are integral to the performance.
Update 2011: Inexplicably, these great recordings are both out of print.