If you still haven’t managed to wrap your own understanding around the credit crisis, take a look at this video, which does a good job of explaining what happened.

… because Porsche had not declared the proportion of VW shares it controlled, traders may have been indirectly and inadvertently borrowing shares from Porsche, selling them to Porsche, buying them back from Porsche [at a higher price] and then returning them to Porsche.