(For instance: recently, "semi-identical" twins were discovered- twins who had come from the same egg, but different sperm. Random stuff like that.)
ETA: Fraternal twins come from two eggs being fertilized from two different sperm. Scientists just discovered a few months ago that it's possible for two sperm to fertilize one egg. http://discovermagazine.com/2008/jan/semi-identical-twins-discovered is the article.