I got a HANG when I try to fix this problem: I can pull up the separate "Pages" mini-window, but when I try to re-integrate it with the left sidebar, Acrobat 9 (on a 2007 iMac running Leopard 10.5.5) gets stuck in a blinking loop. I drag the tab/title "Pages" at the top of the window onto the sidebar, and try to drop. Suddenly, my screen starts misbehaving. Acrobat is apparently trying to erase/redraw the outlines either of the sidebar rectangle (where I'm trying to "drop" the Pages pane back in) or the Pages mini-window that should be disappearing (when I move the cursor away). The application essentially hangs, and the blinking redraw-attempts actually follow me into other applications (Does that mean it's a bug/corruption at the System level, or just that it's an Acrobat problem that reaches all the way through to the OS?)
It seems I can't proceed without force-quitting Acrobat. And alas, when I do, I still don't have the pages function in my left sidebar...
Aha -- UPDATE, for anyone with the same problem. This time (while writing this message), I let the redrawing continue for a long time without force-quitting. About FIVE MINUTES (at least) after the "hang" started, Acrobat dug itself out of its hole and restored its responsiveness (and stopped blinking my screen). Best of all, "Pages" is back on my sidebar where I want it. So, although I'd still call this buggy or corrupted behavior, I found a time-consuming solution.