How long ago was this? What gave it away as being counterfeit? You don't hear about small face bills like this being counterfeited too often these days (unless they're mistaken for being counterfeit out of ignorance) so I am rather intrigued to hear more details about it.
My sister is a teller and she didn't find any security. I also did research and very few things actually checked out. I actually made a thread before I knew it was counterfeit.
Uh huh...I suspect what we're dealing with (short of anything better being presented such as a picture or scan of the note in question, for example) is a classic case of your sister not seeing the expected strip and/or watermark and thinking it's fake even though these features were not yet in existence prior to Series 1990 and Series 1996. I've seen this scenario played out 100 times before where someone thinks a note is fake just because it's old and they don't know any better. The one from your sister is likely just time 101. If by some small chance I am mistaken then please accept my humblest of apologies.
To be honest. The bill is gone, I spent it and I don't care anymore. So whether it was fake or not we will never know
If those bills didn't have watermarks and security strips (in those days), it was probably real. The fake ones usually have dark ink and blurry printing details. And they feel wrong (the paper). If it looked right, and felt right, and just didn't have the security strip, it was probably real.
It is rather common for part of the serial number to show through on the back of notes that are in crisp and otherwise high grade of condition so that observation is not too unusual. The thing that stands out to me is you knowingly spent a note that you thought was counterfeit? I know you likely didn't want to lose out on $20 but offloading what you think is a counterfeit bill onto someone else is not really the right approach to take with this kind of thing...