With Great Knot or Knot?
Striking! Would give me a scare in the field. This wader, found by Paul Lehman on 12th July 2012, had been previously present, nearby, 3 years early in late August 2009, when the plumage was less definitive. So what it is? Opinion (for obvious reasons) centres around a Surfbird which has hybridised with either a Great Knot or (Red) Knot.
Surfbird hybrid (x Great Knot or Red Knot), San Diego Bay, California, July 2012, by BJ Stacey. Lots more photos and video from BJ Stacey of the bird: here.