It can happen a few ways.
Usually a number of sheets of one side are printed, then the whole batch is run through a different press, or the same press setup with different plates.
If a sheet gets stacked upside down the backs will all be wrong and upside down. And usually off center, but not always.
If a sheet gets misfed, and jams it can get the wrong backs on part of it. The other part is usually wrecked.
If there are different sheets - lets say sheet A and sheet B- If a sheet A gets run through along with the B sheets the backs will of course be wrong.
If the plates are made wrong the backs can be in the wrong place. One of the modern sets had that happen, maybe UD black diamond? I forget the year. They corrected it, but some wrong backs were common.
If the print run is large, some modern equipment might almost eliminate wrongbacks. Presses that print from a large roll of stock, presses that print both sides at once, that sort of thing.
Steve B
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