We had gerbils who used to fall over backwards but that was just a one off. We used to give them a small piece of a chocolate digestive and they'd lick the chocolate and go into some form of ecstasy and fall over backwards! Dropping the biscuit so one of the others would pick it up, lick the chocolate and do the same.
Honestly Shades I know it was daft but they all lived long and happy lives. And we didn't do it often. Years back there was no advice against it and it was the pet shop guy who told us they liked chocolate. Mini was nearly 7 when he died. Most gerbils live between 2 and 5 years from memory. And he died because we had to have his teeth clipped. He was too old to be able to keep his teeth pared down and couldn't eat. We knew the "operation" might be too much for him but he couldn't eat so we had no choice.
It really does concern me that chocolate digestives could be given away so easily. I'd fight to the death before I'd give up chocolate digestives, especially the plain ones.