Munster visited the Scarlets with eight wins between them in as many games this season and the Irish province took the early initiative, up 13-6 at the break thanks to Andrew Conway's superb solo try.
However, DTH van der Merwe continued his flying start to life in Llanelli with another try and with the scores level at 22-22 in the final minute, Steve Shingler bisected the posts with the last kick of the game to win it and send the Scarlets top of the league.
Connacht moved second - still level on points with the Scarlets - with an impressive 21-16 win away to the Ospreys the following day.
Man-of-the-match Bundee Aki inspired the Galwegians to victory at the Liberty Stadium, while Steve Tandy's side continue their frustrating start to the season, now with just one win in five.
Edinburgh, who began the weekend in third, slipped behind Connacht after they became Zebre's first victims of the season.
The Italian outfit had failed to pick up a point through the opening four rounds but they found their form at Stadio Sergio Lanfranchi and Dion Berryman's try proved decisive in the end.
Ulster collected maximum points at the Kingspan Stadium, running in four tries to beat Cardiff Blues 24-17 and go level on points with Edinburgh.
Andrew Trimble, Paul Marshall, Nick Williams and Stuart McCloskey each crossed over as the Ulstermen extended their unbeaten home league run to 14 games.
Leinster overturned a half-time deficit to beat Glasgow Warriors in a repeat of the 2014 PRO12 Final at the same venue.
Fergus McFadden's boot was the difference between the two sides at the RDS while youngster Garry Ringrose, deployed on the right wing, also impressed for the hosts.
And Newport Gwent Dragons were made to work hard for their 19-12 victory over Benetton Treviso, Nic Cudd crossing over for the only try of the game.
TRY OF THE WEEKEND
It may have come in a losing effort but Andrew Conway's try for Munster was worthy of winning any match.
When the Scarlets' handling let them down in Munster territory, Conway was quick off the mark and hacked into open space.
The full-back hared after the ball and dribbled it all the way to the Scarlets line, narrowly out-pacing the chasing pack to touch down.
BREAK OF THE WEEKEND
Tom Williams was among the try-scorers as Scarlets went top but his work for DTH van der Merwe's try was equally important.
The replacement picked a perfect line just inside the Munster half and darted through it.
After jinking left and right, he fed Aled Thomas and the full-back had the in-form van der Merwe on his shoulder to dive over.
PLAY OF THE WEEKEND
The frustration goes on for the Ospreys as they were beaten by Connacht on Saturday but Justin Tipuric's try may have offered some respite for their supporters.
And that is in no small part down to the work of Dan Evans after the replacement showed incredibly fast hands to find the flanker.
Tipuric - back with the Ospreys after his Rugby World Cup exploits - then sold a neat dummy to wrong-foot his marker and touch down.
PLAYER OF THE WEEKEND
Connacht's bright start to the season continued with perhaps their most impressive result yet - a first away win over the Ospreys since 2004.
It was certainly a rounded team performance from Pat Lam's men but Bundee Aki was at the heart of it.
As well as creating numerous openings and offering stern resistance in defence, the centre crossed over for a crucial try in Swansea.
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