On a recent trip to Hong Kong, I crossed into Shenzhen China for a day and a half.
I used an ATM at HSBC bank close to the Metro Park Hotel to acquire cash, these were dispensed in $100 bills.
I deliberately went to HSBC as a) it was the bank I held accounts with and b) it is a reputable bank.
When it came to buy a meal at Pizza Hut there was no problem so I was unaware of any pending problems. However, when it came to buy other goods I was told the notes were forgeries but as this was a Sunday I could not make enquiries at the bank.
On my return to Hong Kong I went into a branch of HSBC bank closest to my hotel in Wan Chai, who confirmed the notes were indeed forgeries. I did ask for refund but they gave a slight laugh and said NO. I also gave them my till receipt from the issuing bank but I should have asked for a copy of this.

To this day I have had no explanation from HSBC and no refund either.

To all those cash-strapped and hard-up athletes, and spectators of course, going over to China, be careful of forged currency being issued by HSBC bank.