About halfway down Cambridge street I started to hear an awful noise, so I pulled over immediately.
the bolt that holds the rack to the seat-stay bridge had lost its nut and the whole rack had rotated back 45 degrees until it hit the fender.
While I was wrestling the bolt back through the stay, my co-worker P__ happened to be riding past, and stopped to see if he could help. He had a couple of nuts floating around in his bag, but unfortunately, none of them fit.
A quick solution came to mind, and I detached one end of my rack strap from the axle, looped it through the rack, and then around the seat tube, and then attached it back to the axle. Problem solved!
The weight is borne by the axle anyway, the nut at the bridge just prevents rotation, and the rack strap wrapped tightly around the seat tube kept the rack from moving much.
I was even so bold as to stop at the grocery store and ride home with a fully laden pannier. I didn't break any land speed records, but I made it home just fine, found a nut that fit the bolt, and was certain to soak it in Loc-tite before tightening it back up.