To get a point that was moved incorrectly for whatever reason in a shape key and needs to be reset to the “Basis” position, from Adhi on StackExchange:
Use Blend From Shape:
Select all vertices whose positions you want to reset, Execute Blend From Shape, accessible from W, Ctrl-V or Mesh > Vertices menu, Set “Basis” as shape key to blend from (by default), Set blend factor to the maximum value 1.