If you climb long enough this will happen to you. You reach the top of a climb and the bolted anchors are bad - clearly rusted and maybe even one (or God forbid both) moves around in the hole. Needless to say this is a butt puckering, highly stressful scenario. If you can't top out and walk off then you are at the mercy of those anchors to get down.

Now you have a couple of choices: (1) cross your fingers and pray the anchors hold and get off the climb as you normally would. Or (2) protect yourself as much as possible - which may involve sacrificing some gear.

If you choose option 2, then here are a few tips to minimize risk in this scenario.

• Rappel instead of lower - When rappelling the rope doesn't move though the system. Rope movement (from being lowered by the belayer) may further loosen the bolts from the rock.
• Pull down, not out - Look at how the anchors are positioned relative to the rope angle when you are rappelling or lowering. You do not want the rope to be pulling outward on the bolts. An outward pull might result from having to go over a roof or bulge near the anchor. You may need to leave an extended anchor to maximize the downward pull. Please do not try to save money here by tying webbing and/or using quicklinks in the chains or hangar. This blocks access by others doing the climb (who will not only have to deal with the shitty anchor but now shitty gear blocking the shitty anchor).
• Create a backup system in case the anchor blows - If the anchor blows you will be in a free-fall unless something catches you. There are two ways to create a backup system. 
  • If you have already cleaned the route (i.e. collected all of the quickdraws off the lower bolts) then clip a quickdraw into last bolt before the anchor and clip one strand of the rope into the draw (in the same way as if you were leading so its not backclipped). You could leave a single locker and clip on strand of the rope in, but again, this might screw the next person doing the climb. Since they will be facing the same crap you just faced (and the clip may be a crux) the least you can do is leave them a draw to clip into!
  • If you are on a mixed climb and you already cleaned the route, you may need to place a nut or two (if its a secure placement) or a cam near the top and sacrifice that gear. Again, place one strand of the rope through the draw.
  • If you have not already cleaned the route, you can tie a prusik going from your harness to the strand of rope that is still going through the draws. If the anchor blows then you will fall the distance from the prusik to the draw below you (see the illustration below). This should work whether you are being lowered or rappelling. 

A note on this image: PLEASE NOT NOT LEAVE QUICK LINKS! Quick links are difficult to remove, they get in the way, and make things really difficult for people who get on this route after you. And there will be people who climb the route after you because they may not be aware of the bad bolts! Don't make a bad experience even worse for someone.