This is a fascinating field and reflects human error. The early date stamps/hand stamps were assembled using small pieces (called slugs) and occasionally the postal clerk would insert the incorrect date slug, invert the slug - even place it sideways. They are worth looking out for and most are relativity inexpensive. We see incorrect spellings of town names - and even the wrong number allocated to a town - the most frequently seen is that of Oxford whose correct cancellation number was '613' but for a while used '603' in error - other towns can be found and are scarce to rare. Brighton has a duplex in the incorrect style of a London cancellation and so forth.