League One new-boys Cheltenham stunned a defensively weak Swansea with two early second-half goals.
Butler made an unhappy debut for Swansea
Swans debutant Thomas Butler hit the post in an uneventful first half.
Kayode Odejayi took a cross on his chest and struck a right-footer past Willy Gueret, before the keeper and Kevin Amankwaah slipped up again.
They could not clear Stephen Guinan's cross, allowing Grant McCann to nip in. Leon Knight pulled back a superb diving header, but missed with a late chance.
Swansea: Gueret, Amankwaah, Iriekpen, Monk, Williams, Pratley, O'Leary (Britton 57), Robinson, Butler (McLeod 58), Fallon (Akinfenwa 58), Knight.
Subs Not Used: Oakes, Austin. Booked: Monk.
Goals: Knight 77. Cheltenham: Higgs, Gill, Duff (Wilson 45), Caines, Victory, McCann, Finnigan, Melligan, Armstrong, Guinan (Gillespie 78), Odejayi (Vincent 78).
Subs Not Used: Brown, Bird.
Goals: Odejayi 48, McCann 54.
Ref: S Mathieson (Cheshire).