There are a number of hotels who dont have a 13th floor. Instead it's either 12a and 12b or it goes from 12 straight to 14.
Dont walk under ladders is another, as are, dont pass someone on the stairs, dont put shoes on the table, dont open an umberella indoors, break a mirror for 7 years bad luck. they go on an on and are all things you shouldnt do to avoid bad luck.
Not walking under a ladder seems wise if there is someone on top of it but the rest are old wives tales.