The following list covers the majority of problems that could occur. Electronic equipment in the path of a severe spike can expect to have blown components in the power section. Typically a spike would be caused by either a lightning storm or when power is restored after a failure. While not causing instantaneous damage surges can stress delicate components and cause premature failure. Surges can be caused when heavy machinery such as welding equipment or a lift stops working.
Electrical noise disrupts the smooth output one expects from the mains.
Noise can cause glitches and errors in programs and data files. Many things can cause noise on power lines such as lightning, generators, industrial equipment, fluorescent lights etc. Dips can be caused by the same type of equipment that causes spikes and surges, but voltage changes are induced by switching units on rather than off. A dip effectively starves a computer of power and can cause amongst other things locked keyboards and unexpected system crashes. A blackout is a total loss of mains power.