The Baggies had been in cruise control after Belgium international striker Romelu Lukaku had put them 2-0 up. It could have been more but he hit the woodwork twice.
Reading appeared down and out. They had been distinctly second best and a return to the bottom of the table, after QPR's draw with Tottenham earlier in the day, seemed inevitable.
But Reading conjured a remarkable finish to the game, snatching victory from the jaws of defeat with three goals in the last nine minutes from Jimmy Kebe, Adam Le Fondre and Pavel Pogrebnyak.
The result leaves Reading three points off safety and McDermott believes the teams directly above them - including next weekend's opponents Newcastle - will be nervously looking over their shoulders.
"I think so. We are one game, or three points, from the team above us," McDermott said. "People automatically wrote QPR and Reading off. We have been written off for ages, since we won promotion.
"It is nothing new to me. Since I got the job we have always been written off.
"But we got that first goal and you saw a side that showed real belief and character. That is a game that will go down in lot of peoples' memories."