The various press reports and websites about these mules are mixing their messages. The "dateless coins issued 300 years ago" were not errors, they were intentionally issued without dates on them.
Putting dates on coins is a relatively new concept, in Europe at least. Most British coins prior to the 1600's had no dates on them. Instead, the mintmasters would sometimes place mintmarks on the coins, so the king would know who was responsible for striking which batches of coins, and (roughly) when they were struck. By 1700, all British coins were bring struck with dates on them.
The last regular issue coins to be struck without a date were the silver coins of Charles II, as mentioned by kena above; these would have been struck sometime in the period between 1663 and 1670.
Don't say "infinitely" when you mean "very"; otherwise, you'll have no word left when you want to talk about something really infinite. - C. S. Lewis