WBA v Southampton Play Off Possible Under Rule C.17. Premier League Handbook
If Southampton and Swansea draw 0 0, Swansea lose last game (any score), Southampton lose last game 0 1, WBA win last game 5 2, there could be a play off between Southampton and West Bromwich Albion.
They would have same points, same goal difference and same goals scored.
“Rule C.17. If at the end of the Season either the League Champions or the Clubs to be relegated or the question of qualification for other competitions cannot be determined because two or more Clubs are equal on points, goal difference and goals scored, the Clubs concerned shall play off one or more deciding League Matches on neutral grounds, the format, timing and venue of which shall be determined by the Board.”