Chris Ashton scored a last-gasp try that Owen Farrell converted to save Saracens from a European Champions Cup ambush in west Wales.
The Champions Cup holders needed victory at Parc-y-Scarlets to guarantee a last-eight spot for the sixth consecutive season and a tournament record 14th win in succession.
They failed to get the win, but the dramatic 22-22 draw ended up being enough to send Saracens through in the end thanks to results elsewhere.
The Scarlets looked to have kept alive their hopes of qualifying from Pool Three thanks to Wales centre Scott Williams' second-half try and 17 points from fly-half Dan Jones.
But Ashton struck twice and his fellow wing Nathan Earle also touched down, with Farrell adding a penalty and two conversions, yet the visitors completed a first European game without winning since being beaten by semi-final opponents Clermont Auvergne in April 2015.
They still did enough to claim a quarter-final berth, though, ahead of next Saturday's visit of three-time European champions Toulon.
But the Scarlets' task is far more complex, needing a bonus point victory over already-eliminated Sale Sharks - and also relying on other results - to possibly claim one of three best runners-up spots across five groups, but it appears a remote hope.
Liam Williams lined up at full-back, while his fellow Wales international Scott Williams returned from injury to partner Hadleigh Parkes in midfield and fit-again forwards Tadhg Beirne and Aaron Shingler both featured up-front.
Farrell took over captaincy duties for Saracens from a suspended Brad Barritt - Nick Tompkins filled Barritt's centre vacancy - with Alex Lozowski replacing injured full-back Alex Goode and flanker Schalk Burger taking over from Kelly Brown. A late change saw Earle in for Chris Wyles, who pulled out due to a tight hamstring.
The Scarlets made a strong start, putting Saracens under pressure through a series of forward drives, and they were unlucky not to open their account when skipper Ken Owens squeezed over the line, but referee Alexandre Ruiz ruled a knock-on following lengthy deliberation with the television match official.
Jones then kicked the Scarlets ahead when Saracens conceded a scrum penalty, but the visitors replied impressively following forward pressure, with scrum-half Richard Wigglesworth firing out a long pass to Lozowski, who flicked the ball on and Earle reached out to score.
Again, Ruiz went to the TMO, but a try was awarded, only for Jones to kick a second short-range penalty and edge his team back in front.
With a strong wind at their backs, the Scarlets dominated territory, and a third Jones penalty seemed scant reward for their efforts as half-time approached.
And Saracens, despite Farrell missing a conversion and penalty, knew that a four-point interval deficit was nothing to be alarmed about given that the elements would be in their favour after the interval.
But the Scarlets struck first, capitalising on poor discipline from Saracens hooker Schalk Brits as Jones kicked a fourth successful penalty after securing an attacking platform when Ashton knocked on under no pressure inside his own 22.
Ashton, though, responded to his blunder superbly, finishing off a flowing 50th-minute Saracens move started by Farrell and continued by Lozowski, before Farrell's conversion made it 12-12 to put the ball back in Scarlets' court.
And they hit back immediately, winning quality possession before scrum-half Aled Davies' exquisite pass found Scott Williams, who crossed between the posts in front of watching interim Wales head coach Rob Howley, before Jones' conversion restored a seven-point advantage.
Saracens boss Mark McCall summoned reinforcements off the bench, including giant Australian Will Skelton, and he went within inches of breaching Scarlets' defence, but the home side held out, cleared possession and put Saracens back in their own half.
Farrell and Jones then exchanged penalties as the clock ticked down, and Saracens' fading hopes looked to have finally been extinguished when Skelton was yellow-carded for striking Scarlets flanker James Davies, but Ashton pounced in the game's final move and Farrell did the rest.
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