The most forged notes are the smaller ones because everyone checks the large notes. You can just hand people a photocopy of a €20 euro bill with a hologram sticker and if it’s in a dark bar you’ll probably get to keep the change.

Don't they run all the notes through the checker?

No, that takes way too much time. If you check using a UV light you’ll notice most simple forgeries but I guess most people just don’t expect small bills to be forged.

Does it? Every time somebody pays in cash in a store here they run each bill through a machine that, I'm pretty sure, checks the authenticity of the bill.

A store is not a bar. Stores typically have less transactions but these transactions involve more money, so you can afford more time.

Signs stating "$100 notes not accepted" used to be seen in many shops Canada about 15-20 years ago, when cash was much more commonly used. Since the switchover to polymer bank notes around 2011, counterfeiting has become almost non-existent but cash payments in general are much less common too.

I love how Japan is a cash economy. It's normal to walk around with 2-5 $100-equivalent bills (the 10,000 Yen note) as pocket change for a week or so.

Paid $22,000 in Y10,000 notes for a private event once. The staff didn't even bat an eye.

There's no denomination larger than £50 in England, right? Scotland and NI have £100 which probably adds to the confusion.

In general circulation, that's right - but the Bank of England does have £100,000,000 notes: https://www.bbc.co.uk/news/magazine-21145103

Heh, yep, like to joke that $100 feels like a lot of money whenever you try to spend a $100 bill. I can barely fathom what it's like trying to pass a 500 EUR note.

If you spend time in casinos, $100 notes flow around like water.

Bugsy Siegel look at this with the note of approval

Casinos are also a hotbed of money laundering.