With no home player attacking the ball, Scharner inexplicably headed El-Hadji Diouf's cross into an empty net as keeper Chris Kirkland was coming out to collect.
It left the Latics facing another uphill fight and while Denny Landzaat equalised, Bolton regained the initiative through Kevin Nolan's impudent back-heeled flick before Kevin Davies and Nicolas Anelka completed a 4-1 win.
Anelka could afford to miss a penalty and Bolton were still able to record their biggest win of the season, leaving Bruce immensely frustrated.
"You cannot play in this league and make individual errors like that," said Bruce.
"We made an unbelievable one last week and have done the same again today.
"As a manager, the one thing you cannot account for are individual errors. You have to give yourself a chance and these mistakes are costing us badly."
The defeat, coupled with Middlesbrough's surprise win over Arsenal and Tottenham's victory over Manchester City, means Wigan are now four points adrift of safety, with their top flight status in peril,
"It is not about other teams," he said. "We have to get our own results right.
"We have picked up one point from the last 10 games and if we are going to stay in this league, that has to improve.
"I always knew this job was going to be very difficult. It is a challenge but I am determined to try and put it right."
It was Nolan's quick thinking which allowed him to convert when Anelka's shot squirmed across goal after being blocked by Andreas Granqvist and manager Gary Megson could not have been more gushing about his captain.
"Since I have been here, Kevin has been an absolute credit," he said.
"He sets the standard on and off the pitch. He is a leader in every sense of the word.
"Everyone can see he is desperate to win. He wants his team-mates to win.
"It is not just his goals, it is his work-rate. He is always one of the top three workers in terms of effort. The more of those you can have at a football club, the better you will be."
Megson is still wary about Bolton's position which, while far brighter in terms of the league table, still leaves them only a point ahead of the drop zone.
"You want players to play with freedom but we are where we are," he said.
"At one stage we were rock bottom. Now we are a few places higher but we are still not that far off being rock bottom.
"We have to keep trying to improve both our play and our results."