An indent error occurs when two planchets are inadvertantly fed into the collar, one partially overlaying the other. When the die strikes, the upper planchet is forced into the lower planchet. The unusual indent error below is combined with a double strike: A single planchet was struck but failed to fully eject from the collar. A new planchet was fed into the collar and overlapped the existing one, causing an indent in a previously struck coin, as well as a double strike (hence, the design elements can be seen within the indent).
1908 double struck & 25% indent obverse PCGS AU55