The Commonwealth Period featured year dates on its coinage as opposed to bi-yearly mintmarks which had been previously used on King Charles I coinage. The mintmarks sun or anchor are present on hammered coins but have no significance in terms of the year a coin was actually made. These unfamiliar changes led to mistakes being made when dies were prepared. Sometimes the dies with mistakes were used in production, presumably because no one noticed the errors. Alternatively the dies were either corrected or simply cast aside and the work re-started.
Towards the end of a mint year one sometimes sees dies with wider dates than normal. This was presumably done so that the reverse die could be used towards the end of a year and then modified for use in the subsequent year. Obverse dies were also prone to mistakes which were usually corrected before the die was used, sometimes not.
There are three years where many overdates dominate coinage - 1651, 1653 and 1656. The former and latter years were years when the mint was under pressure and one is left wondering if this pressure led to a shortage of available dies so dies from previous years were modified where necessary and pressed into use. The middle year 1653 was a year when a great deal of coinage was prepared so again more pressure to make do with what was to hand.
Below is an example of a reverse die with an overdate which has its own history. Under the 6 and the 5 in the date one can make out two 4ís. This would lead one to assume that this die was originally handled in 1649, not 1651. Best guess is that these errors led to the die being cast aside. Then in 1651 someone dug it out again and changed the date to 1651 and it was pressed into use.