Red Bull's Formula One leader Sebastian Vettel crashed into the 'Wall of Champions' while Germany's Nico Rosberg set the Canadian Grand Prix practice pace for Mercedes.
World champion Vettel, winner of five of the season's first six races, provided the main drama with 34 minutes remaining when he drifted into the wall coming out of the chicane into the home straight.
While the 23-year-old German was unhurt, the right side of his Red Bull suffered heavy damage.
The session was immediately red-flagged.
Meanwhile Rosberg showed that Mercedes could be a force to reckon with this weekend by lapping the sun-bathed Circuit Gilles Villeneuve with a best time of one minute 15.591 seconds, ahead of Ferrari's Fernando Alonso in 1:16.139.
Seven times world champion Michael Schumacher, who has won a record seven times in Montreal, enjoyed his return to Canada with the third best time, while Ferrari's Brazilian Felipe Massa was fourth.
Vettel was not the only one to have problems, with McLaren's Lewis Hamilton spending time in the garage as the team removed his car's steering column.
The Briton, who took his first grand prix win in Montreal in 2007 and led a McLaren one-two from pole position last year, was sixth fastest behind team mate and 2009 world champion Jenson Button.
Mexican Sergio Perez, who sustained a concussion and bruising in a qualifying crash that ruled him out of last month's Monaco Grand Prix, was back in his Sauber with the 11th best time.
