Most Likely: If you cook the egg thoroughly (to 160 degrees F), nothing will happen, particularly if it cracked after you got it home and you use it immediately. (This is true even if the egg is infected with salmonella.)

Worst Case: A cracked egg is more susceptible to contamination than an uncracked one because the environment inside an egg is more bacteria-friendly than the shell is. A healthy adult who eats a bacteria-infected egg that's not properly cooked will at worst contract food poisoning.