When overclocking your graphics card, a wide variety of errors can occur, but as long as the temperature is under control, lasting damage is very seldom.
The monitor freezes
Most likely, your GPU has hit the limit. This error is not fatal, but can be terrifying the first time it occurs. Besides that, eventual sound may loop.
Lines on the monitor
Your RAM has hit the limit, and you have to decrease the speed.
If pixellation occurs during a benchmark or a game, it is caused by a too high frequency on the GPU or too much heat. The error is seen on figures and elements on the screen get a lot of strange colours, most likely on the edges and in the middle.
Most likely caused by a driver problem. Try to reinstall. Remember to note the error code.
If the screen suddenly becomes black, it can be caused by RAM, GPU or too much heat. This type of error is hard to identify. There is a good chance it is the GPU or heat though.
Your game has crashed, and you can go to the desktop, but the cursor is moving in 5-second intervals. This is because the driver has crashed, and the only solution is to restart your computer.