Or a slightly different perspective of ink registration ... tagging shift errors on block-tagged stamps:
Christmas 1968, Scott 1363. Tagging shifted upward by about 1/4 inch. Same item in normal light and short-wave UV.
Tagging shifted half a stamp to the left. The left column shows a perfectly "split tagging", while the right column shows only half tagging. Portion of a used 2x5 plate strip in normal light and short-wave UV.
Christmas 1969, Scott 1384. Top row with normal and quadruple-split tagging. Bottom row with horizontal or vertical shifts to produce split tagging. Again, normal and UV light.