Kelim, Chapter Seventeen, Mishnah Sixteen
1) The beam of a balance and a leveler that contain a receptacle for metal, carrying-stick that has a receptacle for money, a beggar’s cane that has a receptacle for water, and a stick that has a receptacle for a mezuzah and for pearls are susceptible to uncleanness.
2) About all these Rabbi Yohanan ben Zakkai said: Oy to me if I should mention them, Oy to me if I don’t mention them.
Sections one and two: All of these vessels are various sticks or things like sticks that can contain other things and are therefore susceptible to impurity. In addition, each could be or is generally used for a negative purpose. The beam of a balance that has room to hide metal in it can be used to deceive a buyer by tilting the scales. A leveler is used to level out measuring cups. A person could make a receptacle in the leveler in order to skim the tops of the cups and cheat the buyer.
A carrying-stick with a hidden compartment for money could be used by a worker to deceive his employer into paying him a second time. I.e. he could saysearch me if you find the money on me, then you paid me.
What’s wrong with a beggar’s cane that has a receptacle for water? The Rambam explains that the beggar can claim that he has not been drinking so that people will have pity on him.
The sticks that have a receptacle for a mezuzah or a pearl could be used to cheat a tax collector.
Albeck explains that while all of these things also have legitimate uses, since they can be used deceptively, Rabbi Yohanan ben Zakai laments having to mention them. In other words, he has to teach these halakhot because these vessels can be used in a legal way, but he wishes he could forget about them.