Columbo: "Whew," lifting the victim's wrist, "this is one heck of a watch. Probably cost him two hundred bucks!"
Nearby Sergeant: "Yeah, that's a real beauty!"
Columbo: "But look at that-- the date's wrong! It says May first. My watch only cost thirty dollars, and it has the correct date! Just shows that money doesn't buy quality."
Sergeant: "Uh sir, it really is May first."
Columbo: "Well, what do you expect for thirty bucks."

I think Peter Falk basically adlibbed that entire sequence; the script called for a remark about the date being "wrong" and for the sergeant to correct him on that point, but beyond that I think Falk took liberties. My evidence? First, he apparently adlibbed a lot, to make the interviewees' expressions of surprise and irritation more authentic. Second, the actor who played the sergeant in the above episode-- a mundane, serious character-- let out an out-of-character laugh at that last line that looked and sounded 100% authentic; I think the joke caught him off guard, and he legitimately thought it was funny.