You plug in your crock pot and the kitchen goes dark. Your coffee pot turns off. The lights won't work. Your stove and microwave clocks need resetting. You run to the breaker box, find the switch to the kitchen and flip it. You've fixed the problem, right? The coffee's dripping, the lights work, and you reset the clocks to the minute.
So you confidently plug in the crock pot. Once again, the circuit trips.