About a year ago, I installed some of those little C-SLIDE plastic sliding webcam covers (from @WebcamCovers) on all of our laptops in the house (the kind that are now ubiquitous and private branded by everybody). This week, I had to take one of the laptops in for repair at Apple due to problems with the LCD. There were about a dozen horizontal lines at the top, and a small cone shaped black spot in the middle of the LCD directly underneath the iSight camera. The total repair was over $600 (talk about a markup).
In chatting with the Apple tech (I refuse to call them geniuses), he felt the most likely cause was a pressure crack inside the LCD. Given the machine was only a couple years old, and treated with care, we determined the most likely cause was the added pressure created by the little stick on sliding cov when you close the notebook. Even if you close it gently, the magnets create a pull on the top of the notebook screen. Additionally, even after it’s closed, all of the pressure on the LCD, thanks to the camera cover, is now concentrated on the small area in the center of the notebook, instead of distributed across the entire panel. This means that even while its in your laptop case, any pressure on the lid is focused on one small area of the LCD. The plastic sliding camera covers are very convenient, however it looks as though over the long term, they have the potential to cause severe damage to your laptop screen, even if you care for your machines. I would advise avoiding them and look into solutions that do not interfere with the amount of pressure distributed across the LCD.
As it happens, @WebcamCovers admits that their own products cause damage “when pressure is applied”, however what they don’t tell you is that, even if you don’t abuse your notebook, the “pressure” applied from normal use alone over a prolonged period of time, can cause damage to your notebook’s LCD. In comparison, the little $5 piece of plastic is not worth the risk IMO for a $600 screen. EFF has some good alternatives on their website: stickers that can easily be peeled back and forth, and will re-adhere with no problems. If you care about causing damage to your laptop, I’d recommend looking at this alternative, or others, instead.
NOTE: @WebcamCovers has ignored my request to have the damage caused by their product reimbursed.