Never accept a knife hand-to-hand.
My informant and I were eating rolls at a restaurant when my informant realized that he did not have a knife for butter. I picked up my knife and offered it to him. He looked at the knife in my hand, and then told me that he never accepts a knife hand-to-hand. He then picked up an unused knife on the other side of the table.
Later, when I asked him why he would not accept my knife, he told me that people who exchange knives will fight imminently, like within the next hour or day or so, or, at the least, there will be bad will between them. He doesnt remember ever specifically learning this folk belief, but when he was a little kid he remembers seeing his mother subtly never accepting knives from other people. He tries to be as subtle as she is, saying something like just place the knife here, especially with people he respects or does not know well. He often feels cornered into accepting knives, though, so with peers he is comfortable with, he will just tell them the folk belief.
My informant thinks the folk belief reflects a respect for knives, and the violence that they symbolize. A knife, according to my informant, should not be handled lightly, because it is an object with gravitas, or immense symbolic power. From an etic perspective, this explanation makes sense; knives are dangerous objects and, even in the innocent context of sharing a meal, can connote ill will or violence. The belief might also reflect a respect for guests, family members, and the people that one eats with. Eating together usually implies a peaceful relationship and situation, so by not passing knives around the table unintentionally insulting or implying violent feeling another diner be avoided.