The home side started both halves on top and were rewarded when Chris Dickson volleyed them ahead on 46 minutes from a Stuart Campbell free-kick.
But Yeovil substitute Jonathan Obika headed them level with his first touch on the hour and three minutes later an Andrew Welsh free-kick led to Forbes netting from close-range.
Welsh also provided the free-kick cross for Obika's goal and it was their inability to defend set pieces that cost Rovers the points.
Jeff Hughes should have given them a fifth-minute lead, but shot badly wide of an open goal.
Yeovil started slowly but had chances to win the game more comfortably in the closing stages as Obika and Shaun MacDonald wasted opportunities.