This has indeed happened in a few cases. Secondarily, these hoaxes also tend to dilute the impact and credibility of the few warnings that are legitimate, as the reader cannot quickly separate truth from the more common fiction. It is also amusing to me that the entertainment industry sometimes takes these urban legends and creates a feature using them, which gives creditability to the falsehoods. In the opposite direction, some TV fiction has been accepted as fact, and then becomes part of the email folklore. Forwarding urban legends may also be very embarrassing.
Some hoaxes even mix a grain of truth with an exaggeration to gain believability.
Please, before forwarding anything of this nature, use responsible email etiquette, and check the messages out first. This may save you much embarrassment. This site, updated almost daily, determines the facts, lists the most common hoaxes and urban legends currently in circulation, has a search function, and provides links to many other related sites. Whenever I announce or forward a warning, I include at least one link from a reputable source documenting the accuracy of the statement. Spam falls into different categories that would not surprise any regular email user.