The electric motors in things like refrigerators, vacuum cleaners, washing machines and other whiteware are often sensitive to frequency. Older hairdryers and electric shavers might be also. Even if you use a step-up or step-down transformer, the different supply frequencies mean that motors will run at the wrong speed and quickly burn out. The larger and more powerful the motor is, the more this is true. Don't, for example, bring a vacuum cleaner from the US to Europe (or vice versa). It's almost guaranteed to fail--even if you have a voltage converter.