As long as there have been stamps, there have been unscrupulous people making fakes. Fakes made in days gone by fool even the most expert collector of rare stamps. The latest round is to fake forever stamps and then sell them on EBAY at a discount to people looking to save money on their postage. Personally, I try to stick with smaller denomination outdated stamps for my discount postage, but for many it takes time and effort to match these up and put them on mailings. What follows are a couple articles I picked off of the net.
https://www.linns.com/news/us-stamps-postal-history/2017/february/counterfeit-united-states-flag-and-fireworks-coil-stamps.html
https://www.sellforeverstamps.net/fakes-counterfeits/