I got a nice card from my lawyer the other day. It said "Dear Alan, Just the other day I was walking to lunch and I thought I saw you coming out of the post office across the street. I went over to say hi to you, but it turned out it was someone else, so I turned around and walked back.
My friend, an attorney who has heard all of them, told me this one.
There was a lawyer whose client paid him in cash. One month, the client accidentally gave him twice as much as was owed. The attorney saw the error only after the client had left, but immediately realized his ethical quandary.