Do you know why people tie cans to the back of the wedding car?  According to Mental Floss's 3 Bizarre Wedding Customs Nobody Questioned (Until Now!),

This tradition actually started during the Tudor period in England. As the bride and groom left in their carriage, wedding guests would throw their shoes at them because it was considered good luck if you hit the vehicle. Today that would be considered a lawsuit, so we tie them to the car instead. And since walking home from a wedding with only one shoe is no fun, Americans started using aluminum cans instead.

The other two customs are why we have a "ring finger", and why it's bad luck for a groom and bride to see each other on their wedding day.  Some of the comments are just as interesting as the article, too.