Errors are made by the Bureau of Engraving and Printing every year. The most common of these are perforation errors, either adding an extra row of perfs where they shouldn't be (not an important error), or omitting a row where there should be one (an important error). Coil stamps, that is, stamps issued in rolls, are the stamps that are most often found with missing perforations. This is because their being wound in rolls makes it difficult for the quality inspectors to see whether they have been perforated. Imperforate coils sell for between $15 and $250 a pair, depending on the scarcity of the item. A collector must have a pair to prove that the coil is imperforate. This is insisted upon because a single stamp could just be clipped down to resemble an imperf. Color errors and inverts are far more scarce, but even modern color errors that are scarcer than the Airmail invert have trouble selling for more than about $1,000 per stamp. The Airmail invert has a history of desirability that these other items do not.
Stamp dealers are constantly besieged with calls from collectors and noncollectors alike who have bought something funny from the post office. Most of the items are minor perforation or color shifts. A perforation shift is a stamp where the perforations are not where they should be. A color shift is a misregistration of the colors so that the design appears to be fuzzy. Unless a shift creates a bizarre effect, such items do not find favor with philatelists. But suppose you find a real error, a missing color, or even an invert. Prudent stamp dealers advise clients who do discover such things to exercise extreme patience and care in disposing of them. The reason for this is that there are few Eugene Kleins around any more. Until dealers know how many of a given error will surface, they will be reluctant to pay a great deal for it. In modern times, some errors have come out in huge quantities. Most buyers assumed an error will be common until the cumulative data of a few years proves otherwise.