It looked as though Erik ten Hag's side were cruising to three points in West London, leading heading into the 10th minute of additional time, but late disaster struck.
United had fought back from 2-0 down to lead 3-2, thanks to a brace from Alejandro Garnacho and a late first-half equaliser from Bruno Fernandes.
Former Manchester City man and boyhood United fan Cole Palmer's second penalty of the game levelled matters in the 100th minute though, before the Chelsea midfielder completed his hat-trick barely 60 seconds later to seal a remarkable win.
If the bitter taste of throwing three points away in the final second was not already bad enough for Ten Hag, two more unwanted records have now been bestowed upon the side under his stewardship.
Man United equal 24-year losing record in Chelsea defeatDefeat at Stamford Bridge means United have now lost 17 matches in a single season for the first time since the 1989-90 campaign.
That year, United finished down in 13th in the old First Division, just five points above the relegation zone.
This time around, United are still set for a European finish with a place in the top six likely, but they have now suffered defeat 12 times in the league after just 30 matches.
An early EFL Cup defeat to Newcastle United and four defeats in their six Champions League group stage encounters make up United's other losses this season, in what has been a significant drop off from their performances last season.
One positive omen that the more positive United fans could cling onto is that their side won the FA Cup despite their lowly league finish that season back in 89-90, a
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