"To err is human," and in the complicated process necessary too produce a postage stamp, it is not surprising that mistakes sometimes occur. The artist may make a mistake (As, for example, when he drew an elephant with hind legs jointed the wrong way on a stamp on Sirmoor). The engraver may make some slip in one of the stages of transferring the design to the printing plate.
The printer can easily err in his task, while the man in charge of the perforating machine is not infallible.
Thus we have stamps with the wrong designs, incorrect inscriptions and other faults.
The printer may use a sheet of paper with a wrong watermark, or he may pass it through the press upside down so that the watermark is inverted in relation to the design of the stamp.
If he is printing a stamp in two colours, he may get the part in one colour upside down, giving rise to inverted centres or inverted frames. He may even print the same sheet twice and those are called double impressions. The perforation machine may miss a line, or a whole sheet may be overlooked and escape perforation altogether or be perforated twice.
Overprints and surcharges are frequent sources of error. They are very often set up in a hurry from printer's type, and letters may be missed, wrong letters inserted, the sheet fed through the machine upside down, back upwards, or possibly through twice, giving rise to all sorts of errors.
Varieties are really small mistakes or differences, hardly of sufficient importance to be described as "errors." They are two kinds, constant, where they may be found running through a whole printing of a stamp, or at any rate through a good many sheets, and those which are not constant, happening through some quite temporary circumstance of manufacture and not found in more than a sheet or two.
Stamp catalogues are not very consistent in what they list and what they omit, but the collector will do well to keep any stamps which show small variations of any kind, as a matter of interest, while being careful to avoid the weakness of considering every one of his "minor variety" geese as being of the same importance and value as the "error" swans of the catalogue.