Well, that's if you  leave them in just "slightly longer" than you are supposed to - like MONTHS longer.  Check out this very creepy story.  (Please watch this AFTER a meal only!) 

According to an article at Newser.com, a woman from Taiwan, 23 years old, had the type of contact lenses you were only supposed leaving your eye for one month tops.  She left them in for six months!  Duhhh!!
This is very gross, but here's what happened to this poor girl.
Now I know no one that I have ever met would be neglectful enough to do THAT, but it is still very important to take your contacts out and do what you're supposed to do with them.  I had them for a very long time, and tried to be diligent about cleaning them, but got an eye infection anyway.  (Of course that was back when you had to use solution every single nite and manually clean them with your finger, which is never totally sanitary)
So - there you have it - your warning for the day.