"Neither a borrower nor a lender be" Well! We borrowed from Bill Shakespear and we borrowed, and we borrowed, and we............. still do.
And when I was courting we held a family baby and playfully showed it itself in a mirror. The grandmother went spare "Don't show a baby its reflection until it is a year old". GF (daughter of GM) & I never heard that one!
years later a SO came up with one about "back luck passing on the stairs", now I can see the logic in that, especially on badly lit stairs &/or carrying stuff. Especially in days of yore when carrying a candle wearing a flammable nightdress etc.