I've ended up with fake bills in my possession twice! One was given as change when paying admission at a zoo. We later used that change to pay for food at a confection stand in the zoo and they called security and then the police to come tracks us down and detained us for over 45 minutes while we kept explaining how it was THEM who gave us the bill. Luckily this was a stop at a zoo in toronto on our way to New York on a road trip. We had plenty of cash on hand, I think about $800 in cash. So it was easy to show them all the legit bills so why on earth would we have one fake $5 bill only if we were counterfeiting??
The second time, it was a bill straight from the bank!! Went from the bank to grocery store and they clerk there found a fake when I paid. Again we were lucky. I'd withdrawn $300 from the bank and not shopped at all, gone straight to the store. So was able to show that I had ONLY the $300 on me, and only one fake bill, and it had to be from the bank from whom I had a time stamped withdrawl slip from shortly before paying at the store. I did return to the bank and get my $20 bill reimbursed (they had confiscated at the store, and I felt the bank was to blame. They are a bank for crying out loud and THEY don't check for counterfeit?? They agreed quickly and politely to replace my confiscated $20 after they verified with the store manager that they had kept the bill to give to police).
This time of year is extra bad for getting fake bills. Clerks are so bombarded with long lines and pressured to rush that they simply have no time to scan the bills. I never have heard of pens you can buy for personal use. That's pretty neat