I have seen a lot of time and money spent on a problem just because a ground wire was left loose or a screw was put through a wire, when a previous repair was done. And another problem I see happening a lot more in recent years is, if you want to install a new stereo, CD player, or a amplifier, you must be very careful where you take the power from. Today's computerized cars are very picky about voltage and proper grounds. If you splice into a wire that is in the computer circuit it can reek havoc on your entire electrical system. Also on rare occasions I have seen wires bitten through by rodents of some kind, this really does happen.
Most electrical systems work off of heat.
You need resistance in the light bulb to make it glow. Well there is no real problem with this, but with the increase in lights on the circuit it heats up the flasher much quicker. If a heavy duty flasher is installed it will correct this problem. Electricity like water takes the path of the least resistance. Most electrical problems are rather simple and often overlooked.