I have seen two seprate restaurants that placed a knife ontop of the entrance door's lintel. I was wondering what is the reason for this,I suspect its some sort of segulah,but I am not sure. Does anyone know of such a segulah?
This superstition seems to not be specifically Jewish. See The Penguin Guide to the Superstitions of Britain and Ireland here, which states:
A knife stuck into a mast or a door lintel is good luck.
See there for more knife related superstitions. It should be noted that this version involves sticking the knife into the lintel, rather than placing it on the lintel. Perhaps the version of placing it into the lintel is related to an older superstition; dating back to Pliny, of affixing iron coffin nails to the lintel of door for good luck.