well, next time you'll know, Janie... :D

I find it's more likely to happen when cooking down a chicken or turkey carcass... loads of cartilage waiting to be rendered.

I don't recall it happening with beef, probably because so little cartilage available except in really cheap cuts.

when making stock you can place your selection of vegetables in a large roasting pan and place the bones or seasoned meat on top and then roast in a medium heat oven for an hour or so... add enough water to keep from burning and cover with foil or lid. When the meat is done, deglaze the pan and add the contents of the pan along with the bones and cooked vegetables to a large stock pot... now you are ready to make a rich stock. The roasted meat is ready for the table - but you can take any trimmings and add them to the stock pot. Waste not, want not.