From PCGS:
More excitement came once missing edge lettering dollars were discovered. These are considered major mint errors. Other more minor varieties have surfaced since then, including double overlapped edge lettering, double inverted edge lettering, and weak and partial edge lettering varieties.
Other edge lettering errors in this series reported by PCGS:
2007 James Madison Missing Edge Lettering
2008-P James Monroe Doubled Edge Lettering
2008-P James Monroe Doubled Edge Lettering-Inverted
2008-D James Monroe Doubled Edge Lettering-Overlap
2008 James Monroe Missing Edge Lettering
2008 Andrew Jackson Missing Edge Lettering
2010 Abraham Lincoln Missing Edge Lettering
2015 Dwight D. Eisenhower Missing Edge Lettering