Ireland's Eoin Rheinisch, competing in his third Olympics, finished in 153.98 seconds and 14th place in the kayak slalom after he missed upstream gate 19.
The 32-year-old, from Leixlip, County Kildare, said once he had missed the gate it was "game over".
"A quite measured performance was always the strategy," he said. "I didn't feel like there was a need to take any major risks at this stage of the competition.
"Looking back at the last Olympics, people tend to make small errors that they wouldn't normally make.
"At these huge events people tend to get a bit nervous and a bit jittery.
"I got to that last move, when I went to turn to the right to cross the wave that you have to take to the second last upstream [gate], and the back of the boat touched the bottom and I couldn't finish the turn.
"It just sent me straight on and I just got washed away.
"It was absolute game over. You saw there that there were three of us that took 50 second penalties. It's so unusual for that to happen. It's the pressure of the event.
"The nerves didn't get to me. I am as seasoned a veteran as you can be so it wasn't a nerves issue or anything like that. It was just one of those extremely unfortunate things that I didn't plan for and I don't think you can plan for.
"I was so far away from the bank so to touch the bottom was shock, pure shock and it was over in an instant."