In Japan, shoes are not to be worn in the home. They should also never be worn on traditional Japanese flooring (tatami).
It makes a lot of sense, shoes are filthy.
The worst shoes and the most embarrassing situationWearing shoes in someone's house or on tatami mats in public places is taboo in Japan.
There is one type of shoe in particular that will make Japanese people cringe in disgust — bathroom slippers.
In restaurants (or people's homes) there are slippers in the bathroom. They are provided so that your socks don't get dirty in the toilet. You should use them every time.
Never, ever, ever forget to take the bathroom slippers off before you leave the bathroom. Bathroom slippers are the most dirty objects in the known universe (in Japanese culture).
It's easy enough to forget to take bathroom slippers off. It happens. If it happens to you — people will look at you like you just spit on the floor.