The term "clipped planchet" is probably the biggest misnomer in error coin collecting. The name implies that after or before being struck, a piece of metal was somehow removed (clipped) off. This is not the case. Clipped planchets occur when a thin strip of coining metal is fed into a machine for blanks to be punched out. Sometimes the strip of metal will not be properly fed into the punch, which causes a blank to be punched out that overlaps the spot where an earlier blank was punched out. In the case of a straight clip, the planchet was accidentally punched out near the end of the roll of metal. So, as you can see, clipped planchets are not really clipped at all, but actually improperly punched planchets.

A genuine clip can be told from one made by someone in their garage by two very simple (but nearly impossible to replicate) diagnostics. The first is the metal flow that occurs near the site of the clip, where the extra metal flowed in order to compensate for the empty space. This is characterized by a weakness in the design or lettering near the site of the clip. The other diagnostic of a genuine clip is know as the Blakesley effect (named after the collector that first observed the phenomenon). To tell whether or not your coin exhibits the Blakesley effect, simply check the rim of the coin directly opposite the clip. If it appears the details are lacking in that area and the rim appears flat, then it is more than likely that you have a genuine clipped planchet error.

Very good information. I have never read about the Blakesley Affect.