Number 13.

The number 13 from long time ago is the number of bad luck. There are so much theories about why the number 13 brings bad luck, one of them is because Judas was the thirteen in the table when Jesus died, another is for the norse mithology, another is for the numerology experts says that the bad luck number is the number four, and because the 1+3 it passed to be the thirteen, and so much more theories. 
Thanks to these theories ad superstitions have created so many fantastic stories.And this is why in some many hotels and hospitals the room number 13 doesn't exist. Because much people is supersticious and they're afraid of this number. So in some many buildings the designer has removed this room to avoid problems.