Least Sandpipers (Calidris minutilla), Semipalmated Sandpipers (Calidris pusilla), Western Sandpipers (Calidris mauri) and Sanderlings (Calidris alba) all occur in the US and can easily be confused with each other, especially in winter.
Least Sandpipers, however, have greenish-yellow legs while the others have black legs.

Jamaica Bay Wildlife Refuge, Queens, New York City, New York, USA, 5 May 2014