In Italy it's the number 17 that is bad luck, especially Friday the 17th
Comes from the roman numeral XVII which is an anagram of VIXI which means "I have lived" -a perfect tense in Latin which implies "I am dead..." , at least that's what I've been told. Hence the superstition.
In the DaVinci Code I seem to remember them explaining the 13 thing being linked to a massacre of innocents in France...
A lot of superstitions are linked to nature and death -owls are bad luck, crows, a single magpie etc...
New shoes on the table I would avoid, like Margaret. Not walking under ladders makes sense especially if they are painting up there...
