What are the side effects of overclocking?

Reduced processor lifespan, reduced fan cooler performance over time, bugs.

Explanation:

The reason is that when you overclock your processor you are increasing its base speeds in GHZ. The processor was designed to work at a determined speed, let's say 3.00 ghz. If you increase this speed to 4.00 ghz, it's not just that now it's working faster, it also draws more power from your power supply, and increases the heat that the chip is taking. Processors are designed to endure high temperatures, therefore, you will likely not see any damaged in short term, but your components life span will be severely reduced, also depending on how much you overclock the processor, and the stability of your system, you can see bugs, unexpected restarts, and strange behavior of the computer. As an example, the i5 4670k runs at 3.80 stock speed, it can reach 50 / 65 degrees under full load. If you raise the speed up to 4.5ghz it will reach 70/80 degrees, depending on your ambient temperature and other factors.