Here's a fairly common stock certificate for Long Island Lighting, with a nice vignette. What's unusual about it is that issuing information (name, shares, issue date) were mistakenly printed upside down. This was done either by the electric company or, more likely, by a brokerage. It looks like they caught it right away and voided the certificate; kind of surprising that it wasn't just shredded.
As errors go, this is nothing spectacular. If it were on national currency this would be on par with a signature mistake at the issuing bank. But it occurred to me that in 30+ years of collecting I can't recall seeing any other errors on a stock or bond. You would think American Bank Note would have had a few printing glitches that escaped amongst the millions it printed over the decades. Now that my curiosity is piqued I'm going to start sniffing around.