Starting for the first time in 13 months after injury, Butland denied Brighton's Richie Towell and Jordan Davies in a busy first half at the AMEX Stadium.
But the Seagulls claimed victory in the closing stages when James Tilley's 84th-minute goal was followed by a Tyler Hornby-Forbes strike.
The 2-0 win for Brighton leaves them eighth in the table ahead of their final match of the season, at bottom-club Norwich City on Monday 24 April.
The Potters are second-from-bottom with two matches left, the first of which is at home to West Bromwich Albion on Monday 17 April.
See: PL2 tables