Error stamps. For many, these oddities are the pinnacle of collecting, and they certainly provide some of the rarest and most valuable stamps known. However, certain errors actually only detract from the value of the stamp and knowing how to identify them can be the deciding factor in a smart purchase.

Just 100 of the US' 1918 'Inverted Jenny' 24c stamps were ever found

The first step you need to take is to make yourself aware of the most common types of errors found on stamps and how they happen. The manufacture of stamps can be remarkably complicated and there are hundreds of different ways in which a stamp can be erroneously printed.

You should also be aware that most of these errors are spotted before they have left the printers and are incredibly rare, sometimes with just one stamp in a single sheet affected. In addition, most stamp printers are aware of their value among collectors and have been known to continue printing - or deliberately include - errors in order to gain a larger profit.

• Desirable

Inverted errors

This is where any part of the stamp's design has been inverted, or printed upside down. This is particularly common in two-colour stamps, where each part of the design has to be printed separately, offering the greater potential for mistakes. Perhaps the most famous invert is the "Inverted Jenny" which sell for millions each time a specimen comes to auction.

Colour errors

There are a number of varieties of colour errors, such as albino stamps where the stamp is free of ink. Of course, this is usually accompanied by another stamp from the same sheet, otherwise it would just be a blank piece of paper.

Similar to albino stamps are dry print errors, where not enough ink has been used and the stamp is only partially printed. There can also be colour missing varieties, where two-colour stamps are printed without one of the colours.

Another common colour error is the colour shift, where one or more of the colours on a stamp has been poorly aligned and is printed at an offset to the rest of the design.

Perhaps the most common of all colour errors is simply where the wrong ink has been used, resulting in an entirely different shade altogether.

Perforation errors

These are ones to watch out for, as they are some of the most easily counterfeited stamps on the market. Often, those looking to make a dishonest profit will trim the perforations of a stamp to make it appear to be a less common imperforate variety. When buying an imperforate stamp, it is important to ensure that there are no traces of perforation, so bring your magnifying glass.

You may also come across part perforate stamps, where one or more sides has been perforated yet the others remain imperforate. Perforation shift is when the perforations have been misaligned, causing the stamp to either adopt an odd shape or its main design to be off-centre.

• Undesirable

Flaws

Flaws are perhaps best described as an unwanted error, where the mistake adds nothing to the rarity of the stamp or its appeal to collectors. They are usually caused by tiny defects in the printing plate and are likely to be less desirable when brought to the market.

Good examples of flaws are the marks left by specks of dust and bubbles in the printing ink, known as "doughnuts" which usually occur on the same stamps of each sheet. However, do not write these off completely as they are integral to identifying which plate your stamp originates from and your unwanted flaw can, in extreme circumstances, lead to the discovery of a previously unknown plate. 

Paul Fraser. 

PS. I have some remarkable stamps from all over the world for sale. Click here to take a look.