Since we’re talking about wet and dry printed stamps, now would be a good time to bring up the difference between overinked and underinked stamps. They are not the same as wet and dry printed stamps.
Overinked stamps occur when too much ink is applied to the paper. There are a number of reasons why this can happen. One way is when ink is added to the well that holds the ink for the printing press, some ink may accidentally drop on the printing plate. A second way is when ink is applied to the printing plate, the excess ink is wiped off (using doctor blades). Sometimes the doctor blades are damaged or they don’t wipe the excess ink off correctly. This leaves extra ink behind which is transferred to the stamps.
Underinked stamps occur when too little ink is applied to the paper. This is also called ink starvation. This usually occurs when the printing press is running out of ink and the operator allows the well to run low. Sometimes a solvent will spill onto the printing press which dilutes the ink, giving it a washed out appearance.
Overinked and underinked stamps are not errors. They are considered freaks. Examples with very minor inking problems tend to sell for a few dollars. Examples with very large or drastic problems will sell in the tens of dollars or more depending on the extent of the problem. Overinked and underinked stamps are scarce, but not rare. Keep an eye out for them and you may find a little treasure someday.