If you have to turn the vacuum over and look at the brush roll, turn off the vacuum cleaner and unplug it. The brush rolls on a vacuum cleaner can rip out hair from your head, damage clothes, and injure body parts. Take caution and don't get anything near the openings on your vacuum cleaner.
Extension cords have a wattage listed or an amp rating to help you know what types of things they can power. Too often vacuum cleaners get used with extension cords that are designed for them. Vacuum cleaners can be damaged and fire risk is increased.
Carrying a vacuum cleaner by the cord or pulling the cord are big mistakes, too. Damage can occur quickly to a cord that is being abused. Vacuum cleaners with damaged cords should never be used. The risk of electrocution or fire is just too high. Be sure to inspect your cords monthly to be sure they are safe to use.
Don't use an indoor vacuum cleaner outside. Indoor vacuum cleaners aren't meant to be used on outside surfaces. Similarly, never use a dry vacuum cleaner on wet surfaces.
Don't want to take the time to pick up the penny on the floor? Think again before you vacuum over it. Hard and sharp objects can chip or crack the fan blade on your vacuum cleaner. Don't knowingly vacuum hard and sharp objects. Be sure to scan the room for these objects before you vacuum.
To be safe, make sure that the vacuum cleaner is off when it is being plugged in or unplugged.
It only takes a little bit of maintenance to clean your vacuum cleaner's brush roll, wash/replace filters, and empty the dirt container. But not doing it regularly will ruin your vacuum cleaner.