Coventry City scored a stoppage time penalty to beat Albion 1-0 at the CBS Arena on Wednesday night.
Viktor Gyokeres’ late winner from 12-yards was enough to earn them all three Sky Bet Championship points, compounding the Baggies to their first defeat since October.
Carlos Corberán’s men return to action on Boxing Day at Bristol City.
The boss made one change from Saturday’s 3-0 win over Rotherham, switching Daryl Dike out for Brandon Thomas-Asante.
But, contrary to last weekend, they were on the back-foot in the early stages – Coventry’s Callum O’Hare, Ben Sheaf and Gyokeres all seeing efforts flash wide.
Albion’s first chance came midway through the half when good pressure resulted in the ball falling fortuitously to Tom Rogic on the edge of the box, however his low finish was cleared off the line by a City defender after initially beating goalkeeper Ben Wilson.
Down the other end Alex Palmer had to be at his best to ensure it was goalless going into the interval, saving well from Gyokeres’s near-post strike and Hamer’s long-range swerving shot.
Coventry made a stronger start to the second period, too, with Jonathan Panzo’s header from a corner hitting the crossbar and Palmer once again saving a Hamer drive.
The Baggies, backed by 4,500 supporters, held firm and threatened through a dangerous set-piece, John Swift’s free-kick finding Darnell Furlong only a few yards out, but his nod towards goal bravely denied.
Dike, Jayson Molumby, Taylor Gardner-Hickman and – much later – Grady Diangana were brought on to freshen things up as Albion pushed forward.
Instead it was Coventry who found a goal in added time from the penalty spot, Gyokeres sending Palmer the wrong way to steal the points.
Albion: Palmer; Furlong, O’Shea, Pieters, Townsend; Swift (Diangana 90), Yokuslu, Rogic (Gardner-Hickman 66); Phillips (Molumby 56), Thomas-Asante (Dike 66), Wallace.
Substitutes: Button, Ajayi, Grant.