Dandelions are fascinating plants and well worth looking up online.They can cause calcium problems it's true,if fed to excess,but then the trick is not to feed to excess!They are also diuretic,which means they can make the rabbit pee more than normal,thus risking dehydration.but before you get too worried,this is unlikely to occur if you exercise common sense.They do love them,and I pick them for my rabbits and tortoises,but just keep an eye on the rabbits droppings,and if the greens are causing any abnormally soft motions,cut back and just give as a treat.You will also read that the stems are toxic because of the latex like sap in them,but i queried this with a lady who wrote a book on safe,and unsafe plants for tortoises(which pretty much applies to rabbits and other small furries),and she replied that while there have beeeen a couple of cases in children who have eaten the sap and had a bit of a poorly tummy as a result,she has never known it to be a problem in animals unless you were giving large amounts without balancing it with other plants etc. Common sense is a good guide in most of these things.