Electrons in one transistor are not supposed to be able to reach other transistors in normal circumstances, but according to the principle of quantum tunnelling, an electron can actually escape from an infinitely deep energy well; it just does not happen that often. A transistor is made up of positively and negatively doped silicon around un-doped silicon. Every now and again, through chance alone, an electron can tunnel away from the conductive silicon keeping it in place. Usually it will only burrow in a couple of atoms and then return, though sometimes it can travel into another adjacent transistor. This does not normally cause a problem, because you need a lot of stray electrons to cause an error in how the gate is read. The problems start to occur when an electron attaches itself to one of the silicon atoms in the un-doped section of the silicon, or knocks another electron out of its orbit. This is known as silicon degradation, and over time, usually measured in years, a path is formed by the damage caused by these tunnelling electrons between two gates. Electrons can then flow across the junction freely, causing it to malfunction, and the value be misread by the computer, resulting in an error.
The more energy an electron has, the more likely it is to tunnel, which is why if your CPU is running hot, or has a considerably higher voltage going through it, electrons can tunnel through far more easily. All CPU’s are built so that there is an inbuilt resistance to quantum tunnelling for an extended period of time, but when you overclock your CPU, that period is reduced.
If you overclock or over-volt the chip too much, you can actually physically destroy the silicon lattice of gates within a processor.