The chief side effect of resistors is heat. Resistors drop voltage by absorption. That absorbed electricity has to go somewhere, so it is converted to heat. Engineers design special devices, called heat sinks, to radiate out the heat. Heat sinks are finned pieces of metal placed close to electrical components to dissipate unwanted heat. Without heat sinking, resistors self-destruct. Computers also have small cooling fans, to blow out excess heat created by resistance.
What is wanted in some circuits is unwanted in others. For example, a resistor is used to reduce 12-volts from a car battery to 6-volts for a plug-in cell phone charger. This type of voltage reduction is wanted to properly power the electronic device without frying it. For a power utility grid, a voltage drop is not wanted. Because power lines span for miles, the resistance of the wires eventually drops voltage down. Power companies strategically place substations and transformers, to compensate for the voltage drop. For example, a home needs 240 volts to feed the electrical service panel. The power company may put out 250 volts at the transformer near your house, because they know the wires absorb 10 of those volts due to resistance.
Ohm's law states that resistance equals voltage divided by amperage. Changing the resistance changes the amperage in a circuit. If a circuit's amperage changes, the wires may not be sufficiently big enough to handle the extra amperage. The wires start to heat up due to current overload. If the wires heat up past the point of insulation parameters, the insulation starts to melt. When the wires heat up to the point of setting nearby materials on fire, such as inside walls, framing lumber catches on fire. According to the Electrical Safety Foundation International, over 53,000 house fires per year are the result of faulty wiring.
A resistor drops voltage. If the resistor shorts out, it becomes like a wire, not dropping voltage at all. In the cell phone charger example, 12-volts from the car battery are dropped down to 6-volts for the charger. If the internal resistors in the charger short out, 12 volts are present at the tip of the charger; when you plug the cell phone in, internal circuitry fries, due to excessive voltage levels. Conversely, if the resistor burns out, it forms an open circuit. Instead of 6-volts being present at the tip, zero volts are present.