For thousands of years certain numbers have had a symbolic meaning beyond their mathematical meaning.  In some cultures the number '4' is holy as it corresponds to the four corners of the world; in other, especially biblically oriented, cultures '3' is a holy number (e.g. "the father, the son, and the holy ghost"), but already Pythagoras believed in the magic of numbers including, incidentally, the cardinal number "3".

From the Bible we know the number "666" as a symbol of the Devil, and other numbers are thought of as "unlucky", thus in many American towns there's no 13th Street (the street is there, but usually it's called something else, e.g. "Division") just as most buildings don't have a 13th Floor, but call it "Management" or "Service" or anything else that is neutral.  Same thing for many airplanes.

In modern American English certain numbers carry more than just a symbolic meaning.  The "411" that dials the telephone information service has come to mean "information" itself.  Other numbers refer to specific organizations.

Here's a few to avoid:

Sources: "Hate Springs Eternal," Harper’s, 1 March 2000; "Hate on Display: A Visual Database," Anti-Defamation League (