Openers Josh Cobb and Greg Smith both hit stunning centuries as Leicestershire Foxes beat Somerset by six wickets in a spectacular Yorkshire Bank 40 clash at Grace Road.
Somerset, who had won their first three games in the competition, posted 323 for three with Peter Trego hitting 118 and Marcus Trescothick, Alviro Petersen and Jos Buttler weighing in with half-centuries.
However, Leicestershire responded by scoring 324 for four to win the game with seven balls to spare, thanks largely to the efforts of Cobb and Smith.
The duo set up the record-breaking run chase with the club's highest opening partnership in one-day cricket, a stand of 235 in 25 overs that beat the previous best of 228 by Phil Simmons and Vince Wells against Kent in 1996.
Cobb's 130 was his second successive century in the competition this season while Smith, brought in only because Niall O'Brien was away playing for Ireland against Pakistan, hit a career-best 135 not out.
The Foxes' previous biggest chase came against Durham in 1996 when they scored 289 for victory.
Captain Cobb, who made 107 in the first game of the season against Gloucestershire, again led from the front, reaching a 35-ball half-century with 10 fours.
Smith matched him shot for shot, taking only 27 deliveries for his first fifty, which contained eight fours and a six.
The boundaries continued to flow as the two batsmen put Somerset's attack to the sword, Cobb moving to his century off 62 balls with 16 fours and a six.
The 200 stand came up in the 20th over, but with the record partnership established Cobb was yorked by Steve Kirby.
Joe Burns, Ned Eckersley and Matt Boyce all went cheaply as the Foxes stuttered.
However, it proved only a temporary blip as Shiv Thakor joined Smith in a fifth-wicket stand that produced 46 runs and a stunning victory.
It was difficult to believe Somerset's innings could be eclipsed after some great batting that saw Trescothick and Trego put on 120 in 15 overs for the first wicket.
Trego, eventually caught in the deep off Nathan Buck, hit 118 off 100 balls and the pain continued for Leicestershire’s bowlers after his departure.
Petersen and Buttler contributed unbeaten scores of 63 and 54, the latter scoring at a strike-rate in excess of 200 during an unbroken fourth-wicket partnership of 82 in 7.2 overs.
It was the second time in four games this season that Somerset had topped the 300 total. Incredibly, this time it was not enough.