At some point collectors who buy lots of stamps (as in auction lot, not "Wow, he sure buys a lot of stamps") will find a stamp amidst those purchased that is not listed in the major catalogs.
For the new collector this can be an exciting experience, as the first instinct is to think you've found an unknown variety that is sure to make you very wealthy. Please resist going with this feeling, because it will just make you all the more disappointed when you find that your item is worth 25 cents. And you will.
But it is the .00000000000000000012 percent chance that what you've found is -- if not of a great value, at least a great discovery that will give you something of a name in collecting circles -- that keeps stamp collectors on guard.
Pre-Internet the collector would have to wait for the next stamp meeting, or travel to a stamp dealer to see if a professional would have any information about the mystery stamp. There are a number of possibilities of what you may have. And philately being an area of shared knowledge and information, the answer may be closer than you thought.
Different Kinds of Mystery Stamps
It may be a revenue stamp. If it's a U.S. stamp you may find it in the Scott's Specialized Catalog of United States Stamps and Covers. Such stamps -- tax stamps that paid for everything from taxes on tobacco to playing cards to photos -- are known as "back of the book stamps" for the simple reason that that is where you'll find their listings.
It may be a cinderella. These are many types of stamps that pay no postal duty, yet are found on cards and envelopes as advertisments, cachets or propaganda messages. Their non-postage purpose is usually given away by lack of a country name or denomination number.
A chameleon stamp can confuse the new collector. A difference in shade or color often confuses a collector who believes he has found an uncataloged rarity. Chances are the stamp was left it the sun and simply had its color bleached out. A variation of this is the offset of a stamp design on the back of a stamp. No, it's not a rare "printed on both sides" variety - what you've found is the affect of pressure and maybe some moisture in one stamp on top of another. Simply, some ink came off on the stamp's gummed reverse.
Most perforation anomalies that collectors find on stamps are not errors, but freaks. Sure, if you have a stamp with perforations down the middle of it, you might consider it an error stamp. But a perforation that slightly cuts into the design isn't anything to get too excited about, other than having a good conversation starter.
Same with misregistered colors and ink smears. It must be a dramatic color shift to make the stamp an error. Otherwise, with color just a bit out of borders it is a freak. Ink smears are never errors, but freaks. When it comes to thinking about freaks and errors, the phrase "It should be bad enough to good" applies.
Recognize Imperforate Stamps
Sometimes mistaken for a rare stamp is a cut square from a piece of stationery. Many mistake such items as imperf varieties. There is a good catalog called the Higgins and Gage World Postal Stationary Catalog that can be consulted to clear up any confusion.
Of course there are many imperf stamps around, some errors, some not. Some countries who realize that stamp collectors can be a great source of revenue have been known to print imperfs that are then sold by dealer agents for sums much in excess of the normal stamp. Monaco was known for this, although it has largely curtailed the practice. Some of the Sand Dunes countries, as they are colorfully known in stamp collecting circles, still practice this.
When it comes to perforated stamps remember that rare perf variety stamps can be easily faked. If you find what you think is a cataloged stamp which has a high valuation because of its rare perfs, by all means let a stamp expertizer take a look at it to give you a thumbs up or down.