The pens are worthless in general. Real bills can "fail", fake bills easily pass if they care to. The Secret Service will tell you to not rely on them if you call them up and ask about them.
A friend of mine had a $10 bill checked and confiscated at a corner store a few years ago. The bill was genuine, but a few decades old and over those past decades it accumulated enough starch on it to trip the pen. He had to call the local SS field office and get them to talk the corner store into giving him back his bill (the SS looked up the serial number, said that it wasn't a serial number known to be used on counterfeits, told them that the pens were shit, and told them to give the bill back).
I say the pens are worthless because, although they can detect counterfeits that somebody ran through their ink-jet at home, those sort of counterfeits won't fool a standard "look and feel" check anyway. In fact, they may give you a false sense of confidence and distract you from the more robust "look and feel" check.
"over those past decades it accumulated enough starch on it to trip the pen"
I can't believe that's a very common case. The pens are there to deter people from playing games with their color printers, and do a pretty good job of that.
Well, at least they do a good job of making people buy starch free paper.
A friend of mine had a $10 bill checked and confiscated at a corner store a few years ago. The bill was genuine, but a few decades old and over those past decades it accumulated enough starch on it to trip the pen. He had to call the local SS field office and get them to talk the corner store into giving him back his bill (the SS looked up the serial number, said that it wasn't a serial number known to be used on counterfeits, told them that the pens were shit, and told them to give the bill back).
I say the pens are worthless because, although they can detect counterfeits that somebody ran through their ink-jet at home, those sort of counterfeits won't fool a standard "look and feel" check anyway. In fact, they may give you a false sense of confidence and distract you from the more robust "look and feel" check.