Le Golf National, venue for the 2018 Ryder Cup, continued to cause major problems for some of the world's best players as the Alstom Open de France continued on the outskirts of Paris.
Dropped shots had been more common than birdies on the opening two days, with the course's daunting closing stretch surrounded by water wreaking the most havoc.
But the opening holes also claimed some notable victims as the leaders set out for their third rounds today to leave the tournament wide open.
Overnight leader Fabrizio Zanotti had made the ideal start with two pars followed by a birdie on the third, but the Paraguayan then carved his second shot to the fourth into the trees and was forced to declare his ball unplayable.
After trudging back down the fairway to hit another ball, the Paraguayan's fourth shot found a greenside bunker and the resulting triple bogey seven dropped him back into the pack on four under par.
South African Richard Sterne had been alongside Zanotti on seven under after birdies at the first and third, but bogeyed the fourth and ran up a double bogey on the fifth after chipping from one side of the green off the other and into a bunker.
That meant Denmark's Thomas Björn found himself in the outright lead, despite having a double bogey of his own at the second where his tee shot found the water short of the green.
Björn had birdied the first and recovered from the double bogey with birdies at the next two holes to reach six under par, one clear of Graeme McDowell who had enjoyed a more serene start.
McDowell, who has won twice in his last seven events but missed the cut in the other five, had birdied the third and registered five pars to be five under.
Zanotti admirably regained his composure immediately and holed from long range for a birdie on the fifth, only to then slice his drive out of bounds on the seventh to suffer another triple bogey.
In contrast, playing partner Björn enjoyed a slice of luck when his tee shot to the eighth took a good bounce on the edge of the green and rolled towards the pin, leaving him just a foot away for birdie.
That took Bjorn to seven under par and a two shot lead over McDowell, who had been unable to get up and down from just left of the ninth green for birdie.
It was McDowell's turn to come close to a hole-in-one when his tee shot to the 11th pitched inches right of the flag, a birdie there taking the World Number Nine to six under par.
And when Björn bogeyed the same hole 20 minutes later after going over the green, the pair were tied at the top of the leaderboard.
Austria's Bernd Wiesberger completed an excellent 68 to set the clubhouse target on four under par, a target soon matched by England's David Howell courtesy of a 69.
Björn missed good birdie chances on the 13th and 14th to remain six under alongside McDowell, who had safely parred the dangerous 15th and 16th where playing partner Simon Dyson dropped shots to fall back to three under.
