When a die become dirty, the devices, which are incuse on the die's surface, can become filled with dirt or grease. When this happens, the part of the die that is clogged will not leave a raised image on the coin's surface when it's struck. The image that is pressed into the die has tappered sides, so as it begins to fill with debris, the image that is left on the coin's surface will appear wider. As the die image continues to fill, the area will eventually become full of debris and fail to transfer any design to the coin's surface, resulting in missing details as shown in the examples below.
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The image above (submitted by Sandy Peters) is a 1927-D cent missing the AM from AMERICA.
Errors like the last two shown above can be confusing at times. Grease filled dies or low pressure strikes? Since the rim is formed during the strike, and these examples have full rims, they are struck through errors and not weak strikes.