Mislaid things have been lost, but only temporarily. You might end up finding your mislaid sunglasses in the last place you'd think to look — like the refrigerator.
When you mislay something, you put it down in the wrong place. Mislaid things will almost certainly be found again, but it can be frustrating when something important like your homework or your car keys are mislaid. Mislaid and mislay use the "bad or wrong" prefix mis-, along with lay or laid, "put down."