You Must Not Cut Your Nails at Night
If you do, you will not be present at your parents death. That is, you will die before your parents die. There were no electric lights in the old days, so it was dark at night. So of course, it would have been dangerous and foolish to cut your nails in the dark. You could cut off your fingers!

You Must Not Sleep with
   Your Head to the North

Kitamakura'. Kitamakura is associated with the dead. According to busshiki (Buddhistrites), the dead body should be laid with its head to the north. So if a living person sleeps this way, it could be very unlucky.

When You Make 'Sayu', Hot Water
You must not add to hot in water. Because in Japan hot water, '
Sayu' which is offered to a family alter we add to hot in water. So it is unlucky.

You Must Not Give Foods from
   Chopsticks to Chopsticks

Hashiwatashi', To pass foods from chopsticks to chopsticks is called 'Hashiwatashi' in Japan.
When people pick up bones after cremating the dead body, they give bones from chopsticks to chopsticks. It is said that it's not good behavior.

When Milk Teeth (Baby Teeth) Come Out,
   the Upper ones should be Thrown Over
   the Roof

Throwing the upper teeth over the roof symbolize the hope that the new adult teeth will grow straight and healthy.

Don't Whistle at Night
If you whistle at night, it means a thief will come to your house.
In the
Edo period, the whistle was associated with detectives who would patrol the streets at night and use the whistle as an alarm signal.