I'm spending a lot more time with early music, and as I was studying the score to J. S. Bach's third Brandenburg Concerto, I encountered something unexpected: an entire Adagio movement that's only one measure long (listen here):
Wikipedia says that
although there is no direct evidence to support it—it was likely that these chords are meant to surround or follow a cadenza improvised by a harpsichord or violin player.
And a page by the Redlands Symphony says
The second movement of the concerto is something of an enigma. It consists of two lone chords, with a fermata (a "hold") over the second. Did Bach intend for the performers simply to play these two chords and then move on to the third movement? Or did he intend for one or more of them to improvise a cadenza elaborating on the transition? Musicologists and performers have expressed varying opinions regarding this question.
Can anyone shed any light on this? Does this correspond to any earlier concerto practice, or is this completely out of left field?