I've had to handle cash a bit over the last couple of weeks, mainly on volly Fridays, and it took me a week or two to get used to the coinage again. I only made one mistake though; gave some a £2 coin in change by mistake for a £1.
There is actually a serious point to this (otherwise it would just go in the Div thread). Notes are clearly marked with their denomination on both sides, but on coins it's much less clear. The markings are smaller, they tend to be one side only, and they're sometimes in numerals, sometimes in words. We really rely on familiarity to distinguish them.
Unfortunately the same problem can even apply to card payments. In certain lights or at some angles, the display on the payment terminal can be unclear.