Alright, I guess they say you shouldn't believe everything you hear, but when it comes to hygiene, it's usually better to be safe than sorry. Case and point: Lian Kao, an undergraduate from Taiwan, left the same pair of contact lenses in for 6 months straight and is now blind.
Microorganisms called Acanthamoeba, who thrive in deoxygenated surroundings (which is exactly what the space between a contact lens and an eye is), begin to feed on the bacteria that builds up on the eye from the lack of cleaning and taking care of it.
Eventually, as the bacteria becomes more deeply burrowed into the eye, the Acanthamoeba begin to feed on the cornea and deteriorate the sensitive human tissue that we rely on for sight.
The condition that Kao developed is called Acanthamoebic keratitis. It can occur for a multitude of reasons - all leading to the fact that the contact lens was not disinfected prior to beinng used. It's not all customer-caused - Advanced Medical Optics (AMO) recalled a large number of their products because of the potential danger that their customers faced due to a failure in properly disinfecting the products to protect from the condition.
Of course, if the product arrives dirty and with a high chance of infection, that is out of the customer's control. However, it is far more controllable and far more important to make sure that contacts are put in and taken out in timely fashions. Kao left her contacts in for half a year - she wore them when she used the bathroom, when she slept, and when she swam (which is terrible in itself because swimming pools are generally known for containing Acanthamoeba).
There's a chance of recovering from this condition through corenal transplants, but even this is iffy because once the infection is in there, it's extremely difficult to get it out.
So, people, the lesson here is take care of your eyes. Listen to your 30-day or one-a-day contact box and don't question the authority of the medical world on this one.