Tatami in the summer time
Although not all houses have tatami flooring these days (many apartments and homes will just have one or two "tatami rooms", usually bedrooms), you generally do not have a bed with legs on the tatami floor. Having heavy furniture with edges will leave pressure dents in the tatami after time, so simple soft furniture is generally preferred. Most will of course use a futon on tatami flooring, and you have to be careful in the summer.
You can sweat through your futon and combined with the humidity, can turn the tatami flooring under your futon into a bamboo marsh. With the Japanese humidity, this will leave mold on your tatami floor, so it is best to pick up your futon every couple days, and hang it out your window to air it out.