Albion saw their 14-match unbeaten run come to an end as they tasted league defeat at The Hawthorns for the first time since February with a 2-0 reverse against Middlesbrough.
Daniel Ayala's 17th-minute header from Lewis Wing's corner separated the sides before Ashley Fletcher added a long-range second for the visitors in injury time, as the Baggies fell to their first loss since October and only their second of the season in their final match of 2019 and last of the decade.
Albion also saw their 17-match unbeaten run on home soil in the Sky Bet Championship, the second-longest in the club's history, come to an end.
Yet the Baggies will be looking to make immediate amends on New Year's Day in a top-of-the-table clash against Leeds United.
Slaven Bilić made four changes from the 1-1 draw at Barnsley on Boxing Day, fit-again Nathan Ferguson returning alongside Gareth Barry, Kyle Edwards and Matt Phillips.
Although it was the visitors who opened the scoring with the game's first attempt on target after 17 minutes, Boro skipper Ayala heading home Wing's corner.
Barry blocked from Wing minutes later before Sam Johnstone kept out his follow-up strike, with Marcus Tavernier clipping the crossbar with a cross shortly afterwards as the visitors applied pressure.
Furlong headed over from Pereira at the other end, before the Brazilian tested Aynsley Pears with a far-post volley and George Saville forced Johnstone to get down low as half-chances came for both sides.
Edwards was next to go close with a curled effort beyond the far post while Wing again drew a save from Johnstone, this time from distance, as Boro led at the break.
Bilić introduced Charlie Austin and Conor Townsend at the interval and immediately saw his side on the front foot, captain Jake Livermore's effort tipped over by Pears.
Filip Krovinović was introduced on the hour mark as the Baggies continued to press in search of a leveller.
But it was Johnstone who was called into action to tip over from Tavernier in the 67th minute.
Austin was denied by Pears as the encounter entered the final stages while Johnstone kept out Boro substitute Marvin Johnson.
Bilić's side continued to push forward but it was Fletcher who doubled Boro's tally with a strike from fully 35 yards in the 94th minute, as Albion drew a blank for the first time since their only previous defeat this term in October, ending the club's longest unbeaten sequence since 1957.
Albion: Johnstone, Furlong (Townsend 46), Ajayi, Hegazi, Ferguson, Livermore, Barry (Krovinović 59), Phillips, Pereira, Edwards (Austin 46), Robson-Kanu.
Subs: Bond, Bartley, Sawyers, Brunt.