There are many laws in football, as the official who fell foul of Sunderland's "balloon goal" discovered to his cost at the weekend, and one in the unwritten category dictates that forwards discarded by clubs often return swiftly to hurt them.
So it proved here as Craig Beattie, sold by West Brom for £500,000 in August, returned to the Hawthorns to net a deserved winner for Swansea, who have now gone seven games unbeaten.
The Scot managed just seven goals in two seasons as an Albion player, large portions of which were spent on loan elsewhere, but there was no disputing the quality of his 49th-minute decider, which was hailed by the noisy away fans and left the home support having to resort to bitter jibes about the beefy forward's weight.
Roberto Di Matteo's side stay top of the Championship after Newcastle lost at Scunthorpe. The two sides are on 24 points with Albion's greater goals tally the difference. Albion had goalkeeper Scott Carson to thank for that as he made a series of saves in a contest delayed by traffic congestion.
His first foiled Nathan Dyer from close range after Beattie had muscled his way into a crossing position, another kept out a header from another Albion discard, Ashley Williams, and late on he deflected a potential clincher from substitute Thomas Butler on to a post. Carson had no answer to Beattie's strike, with the forward, having been put in space on the left of the box by Mark Gower, firing across him to find the bottom corner.
The result was the most impressive of new Swans manager Paulo Sousa's reign. "It is one important night for me as I can recognise we are in a good way," he said. "We need to follow that way."
True, the youngest man on the pitch, 17-year-old striker Chris Wood had almost put Albion ahead, volleying over at full stretch and Luke Moore's loft over Swans goalkeeper Dorus de Vries dropped just wide of a post.
But it was substitute Simon Cox who wasted Albion's best opportunity by firing wide seconds after coming on, although Wood headed tamely wide in stoppage time.
Albion assistant manager Eddie Newton said: "We were hit with a sucker punch. We were naive and they were very professional."
West Bromwich Albion (4-4-2): Carson; Zuiverloon, Jara, Olsson, Mattock (Cox, 74); Thomas, Dorrans, Mulumbu, Cech (Teixeira, h-t); Moore (Koren, h-t), Wood. Substitutes not used: Kiely (gk), Reid, Meite, Martis.
Swansea (4-4-2): De Vries; Rangel, Williams, Tate, Painter (Richards, h-t); Britton, Pratley, Dyer (Butler, 61), Gower (Allen, 74); Van der Gun, Beattie. Substitutes not used: Cornell (gk), Lopez, Trundle, Idrizaj.
Referee: A D'Urso (Essex).