Guess it can happen but many probably don't know of that trap.
There's a bigger trap than that on the GW parkway. All of Columbia Island, which the parkway traverses, is part of DC. See pic below.
He made a wrong turn and ended up going across Memorial Bridge. When he got to D.C., he immediately made a U-Turn and headed back to Virginia,
Actually, he would have been in DC before he ever got to the bridge. Both ends of the memorial bridge are inside DC: