The 31-year-old Scotsman came to the 563-yard final hole at Royal Mougins four under par and after two great shots was left with a 60-foot eagle chance.
If it had gone in Montgomerie would have finished the opening round of the £300,000 event just one stroke behind France's Jean Van de Velde and Sweden's Mathias Gronberg. Instead, though, the ball picked up speed going down two big tiers in the green and sped past the hole into the waiting water 20 feet further on."
Europe's leading money-winner for the last two seasons eventually walked off with a bogey six rather than the four or even three he was hoping for and had to settle for a three-under-par 68.
"I've never done that before and I hope I never will again," reflected Montgomerie, who even managed to smile about the incident. "These things happen. No they don't - what am I saying?"
He had been five under with three to play, but hit his tee shot into a hazard at the 419-yard 16th and dropped a shot there as well. "Twice in the water in three holes. Once with a driver, once with a putter. Apart from that I'm playing great."
The defending champion and tournament favourite, Ian Woosnam, managed to avoid the eight lakes, streams and waterfalls on the new course and came in with a five-birdie 67.
The Welshman, making his first European Tour appearance of the season, continued to experiment with the left hand below right putting method used now by Nick Faldo, but said: "I've not decided whether I like it yet - it feels uncomfortable."
The best rounds of the day were the seven-under-par 64s of Van de Velde and Gronberg. Van de Velde's score included a penalty drop after going into the water at the long fourth. The 28-year-old saved par with a 30- foot putt there, reached the turn in a three-under 32 and then had four more birdies coming home.
AIR FRANCE CANNES OPEN (Royal Mougins, Cannes) First-round scores (GB or Irl unless stated): 64 J Van de Velde (Fr), M Gronberg (Swe). 65 A Bossert (Swit). 66 D Hospital (Sp), O Sellberg (Swe), J Bland (SA), A Coltart, M Besanceney (Fr). 67 J Lomas, R Boxall, I Woosnam, J Robson, A Binaghi (It), N Fasth (Swe), O Rojahn (Nor), M Roberts, E Darcy. 68 R Chapman, I Palmer (SA), A Hunter, S McAllister, E Giraud (Fr), D Robertson, P Moloney (Aus), C Montgomerie, P-U Johansson (Swe), M Harwood (Aus), J Haeggman (Swe). 69 J Sandelin (Swe), P Hedblom (Swe), I Garrido (Sp), M Campbell (NZ), P O'Malley (Aus), M Pinero (Sp), E Canonica (Italy), F Nobilo (NZ), P Fulke (Swe). 70 M A Martin (Sp), M James, D Gilford, S Torrance, R Drummond, M Miller, S Cage, R McFarlane, M Roe, G Turner (NZ), R Rafferty, S Tinning (Den), D Curry, P Mitchell. 71 P Way, C Cevaer (Fr), R Karlsson (Swe), S Luna (Sp), R Goosen (SA), G Orr, J Bickerton, J Remesy (Fr), S Struver (Ger), P Broadhurst, J L Guepy (Fr), B Lane, H Clark, P McGinley, J Payne. 72 R Claydon, J Townsend (US), J Robinson, J Quiros (Sp), F Tarnaud (Fr), A Gillner (Swe), P Golding, I Pyman, M Hallberg (Swe), L White, D Clarke, M Lanner (Swe), A Murray, P Eales, V Fernandez (Arg), M Archer, D Ray. 73 M A Jimenez (Sp), T Johnstone (Zim), D J Russell, S Grappasonni (It), K Eriksson (Swe), M Mouland, P Simpson, R Lee, P Fowler (Aus), B Telleria (Fr), G Levenson (SA), M McLean, D Smyth, W Riley (Aus), T Bockmann (Nor), O Karlsson (Swe). 74 J Spence, F Jacobson (Swe), A Lebouc (Fr), T Levet (Fr), C Rocca (It), F Regard (Fr), *M Dieu (Fr), P Quirici (Swit), S Bottomley, M Clayton (Aus), D Williams, M Sabri (Mor), P Walton, A Cejka (Ger), E Dussart (Fr), P Curry, J Hawkes (SA), M Gates, A Oldcorn. *denotes amateur.