It should also be noted that it would appear that during some mints may have deliberately used retrograde letters.
During the final emission of Probus from Lugdunum they chose to use A, B, C and D as their 4 officina identifiers. Officina B, C abd D all produced many dies with the officina letter in both normal and retrograde form. The number of dies with retrograde letters is smaller than the standard direction but are also too numerous to think that it is simply an isolated set of errors. We can only suppose that they were using retrograde letters in this mint at this time for some meaning that we cannot know as it was never documented.
I think that most such retrograde letters are simple engraving errors. Z seems to have been a regular error.
A more peculiar retrograde error is illustrated on the following coin. The engraver was attempting to engrave VI and engraved the pair as IV (IV is not used as an officinal mark).