Albion fell to a 3-2 defeat against Sky Bet Championship promotion rivals Middlesbrough as Britt Assombalonga scored twice late on at The Hawthorns.
Jay Rodriguez and Dwight Gayle struck either side of half-time, each registering their 15th goals of the season, to put the Baggies in front just after the hour mark, coming after George Saville had pounced to put the visitors into the lead.
But Boro substitute Assombalonga levelled matters 15 minutes from time before firing a winner with seven minutes remaining as Albion tasted only their second defeat in 15 matches in all competitions and first on home soil since October.
Albion showed one change from the league victory at Bolton Wanderers, with Sam Field, who scored off the bench in Lancashire, named in the starting XI, while new recruits Jacob Murphy and Jefferson Montero were among the substitutes.
Gareth Barry, wearing the captain's armband with Chris Brunt and Jake Livermore on the bench, almost gave the hosts a dream start after just three minutes but his low shot was tipped around a post by Darren Randolph.
Yet it was Boro, managed by former Baggies boss Tony Pulis, who took the lead in the 17th minute as Saville won possession in a dangerous area, drove forward and drilled low into the far corner.
The goal checked Albion's stride and saw them lose their early rhythm, struggling to find a cutting edge despite continuing to dominate proceedings.
Yet Darren Moore's side got the breakthrough they deserved three minutes before half-time as Rodriguez pounced at the back post to convert Rekeem Harper's attempted shot.
Harper tested Randolph minutes after the restart before dragging another effort wide of target as the resurgent Baggies flew out of the traps at the start of the second period.
Sam Johnstone saved low from Jordan Hugill at the other end before the Baggies took the lead just after the hour mark, the ball falling on the edge of the penalty area to Gayle, who controlled and sent an effort beyond Randloph and into the far corner.
Murphy was introduced off the bench for his Baggies debut in the 75th minute.
However, Boro were level moments later as sub Assombalonga created space and drilled low into the net from inside the area.
Moore turned to Livermore and Brunt in a bid to force a winner, but it was Assombalonga who struck seven minutes from time to secure the spoils for Boro.
Albion: Johnstone, Holgate, Dawson, Hegazi, Gibbs, Harper, Barry (c) (Livermore 78), Field (Brunt 83), Robson-Kanu (Murphy 75), Rodriguez, Gayle.
Subs: Bond, Adarabioyo, Bartley, Montero.