It was Tony Pulis who crushed Tottenham Hotspur's Premier League title dream last season but Dele Alli left it late for payback.
Spurs's unbeaten record stretches to eight games after Alli's 89th-minute equaliser, on a day when Pochettino must have been fearing the worst against party-pooper Pulis for the second successive time.
Nacer Chadli, West Brom's pounds 13?million record signing, had struck in the 82nd minute against his former club to leave Spurs facing yet another potentially damaging result, with Pochettino's day of agony only increasing after an injury to Toby Alderweireld.
The Belgian defender was stretchered off early in the second half, after a collision with Jan Vertonghen, and is unlikely to travel for the Champions League game against Bayer Leverkusen on Tuesday.
And though Alli made amends for an afternoon of profligacy with the leveller, the injury to Alderweireld could still prove a pivotal moment in Spurs's season.
This was the first of seven games in 23 days and Pochettino will be anxiously waiting for a positive outcome.
"With Toby we need to wait 24-48 hours. He got a heavy knock in his leg. We will see what happens, now it is very painful. I asked him how he was and he explained he had pain," he said.
"It is always difficult against West Brom but we showed great maturity to stay unbeaten, which is important for our confidence.
"We missed an opportunity to get three points but this was a good message from us."
While Spurs were scintillating against Manchester City in their last game, bloodying the nose of Pep Guardiola for the first time in the Premier League, this was a hugely frustrating experience.
With West Brom adopting the Pulis formation and organisation to the letter, Spurs were strangely unconvincing. They also had to endure a masterclass from Albion goalkeeper Ben Foster, who took centre stage late in the first half.
Foster dived to his left to prevent Ben Davies's low drive from finding the corner and then denied Alli after he broke clear of Albion's defence. Three minutes before half-time he produced arguably the most impressive save as he pushed away Christian Eriksen's fierce shot.
"England's No. 1" chanted the Albion supporters, as Pochettino fidgeted furiously in his technical area.
The Spurs head coach was even more unsettled on the hour when Alderweireld collided with Vertonghen in the penalty area, falling awkwardly to the floor.
Pochettino introduced Son for the final 18 minutes but it was now Albion in control and a goal was inevitable. It finally came eight minutes from time, from a predictable source, as Chadli finished from close range after Lloris pushed James McClean's shot onto the upright. He refused to celebrate but the Hawthorns went crazy, a famous victory in sight.
Yet Alli had the final word of an absorbing climax, stabbing in from close range to ensure Spurs have now gone eight league games unbeaten for the first time since 1990.
Pulis said: "They are one of the best in Europe, never mind England, it is wonderful that you can watch teams like that with such an array of talent, and that's because we're in the Premier League.
"I don't think anybody should take it for granted, especially clubs that are outside the top six or seven in this country. If you look at the whole game we have Ben to thank. It's about time he made a few saves, he's not had a lot to do this season."