Summarize this article:

Older rabbits might not be able to curve their spine properly, and could end up soiling themselves during urination.  Check its fur, especially its back legs, for wetness or fur clumps due to urine.  If the problem is chronic, your bunny can develop urine scald, a condition which causes the rabbit to suffer from inflamed or irritated skin and fur loss. If your pet develops urine scald, ask your vet for an appropriate treatment which you can apply to the inflamed area.  There are several available, but they generally work to relieve pain and fight infections.  Ensure your cream, powder, or ointment is zinc-free. Keep your cage clean and ensure your cage’s top layer of bedding works to wick away moisture from the surface.  Check your pet’s bottom regularly.  If you see an accumulation of fecal matter or urine stains on its hindquarters or around its anus, give it a bath and contact your vet about the situation. If your rabbit has thick or pasty urine, it might need a reduction in its calcium intake.  Identify sources of calcium in its diet and find an appropriate substitute for the other nutrients which might be lost when removing the source of excess calcium.  For instance, if you feed your rabbit a helping of food pellets with each meal and they contain 100% of your rabbit’s daily value of calcium, reduce the pellet serving by half, and find another type of pellet which is calcium-free to make up for the other half of your rabbit’s calories in each meal.

Summary:
Pay attention to bunny’s potty time. Check the cage for a foul smell indicative of urine or feces. Ensure your rabbit is getting the right amount of calcium.