In the 2010 PGA Championship at Whistling Straits, players were asked to treat every piece of sand found on the course as a bunker. Dustin Johnson didn't on the 72nd hole, and it cost him the championship.
Ambiguity about what does and what doesn't constitute a bunker will likely be an issue these next two weeks at the U.S. Open and U.S. Women's Open at Pinehurst No. 2.
According to Golf World's Tim Rosaforte, a 3-hour USGA Rules Committee meeting on Wednesday at Pinehurst No. 2 did not resolve the true definition of a bunker at the renovated Donald Ross gem. As part of their restoration work, Ben Crenshaw and Bill Coore removed all rough, replacing it with sand-based hardpan areas with interspersed native grasses growing in tufts. However, they didn't remove bunkering on the course, leaving some degree of uncertainty about where players can ground their club when their ball lies on top of sand.
In a 2011 round at Pinehurst No. 2, resort president and CEO Don Padgett II told me bunkers would be defined as areas where native grass completely surrounded the sand. The emphasis is on "completely" as there are a number of areas on the property where that's not the case but could otherwise be construed as a bunker.
It's likely the USGA will tell players to treat everything as a bunker and as the rules official assigned to each group for help in deciphering the bunker enigma during play.