Last year's runner-up coasted into a 3-0 lead, scoring 278 unanswered points before an "embarrassing" collapse.
But Trump's defeat was overshadowed by a heartbreaking missed 147 by Thailand's Thepchaiya Un-Nooh.
Un-Nooh fluffed a maximum break with a final black miss in the sixth frame of his 6-2 defeat against Neil Robertson.
The miss cost Un-Nooh £44,000 in total.
Robertson, the 2013 champion, made a 145 in the next frame to claim the highest break of the tournament and eased to victory.
Un-Nooh's gut-wrenching error, after several astonishing pots had kept his hopes of a maximum alive, partly kept Trump's troubles out of the limelight.
Trump, the 2011 winner, scored 112 in the first frame, 75 in the third and looked in majestic form to lead 3-0,
But a kick in the sixth frame, when he could have gone 5-1 ahead, changed the course of the game according to the pre-tournament favourite.
"At 4-1 I felt in complete control and was playing well, but I had a kick and it was embarrassing towards the end," he said.
"It was one of those games where I didn't want to be out there any more. I didn't think I could pot a ball. I don't know what went wrong. It's worrying.
"It is probably the worst I have ever felt as a professional. I was never going to win. It was very disheartening. I knew something would go wrong. It was a horrible feeling."
Wenbo said: "I am very happy. At 4-3 I thought I had a chance. He missed some easy balls."
In the afternoon session World number four Murphy comfortably beat Ben Woollaston, finishing an increasingly impressive performance with a stylish 106.
The 2008 champion began in ominous fashion, hitting an opening-frame 83.
Woollaston had several chances but Murphy showed his matchplay quality and potting ability to score two 70-plus breaks and secure victory with a 106.
"I am very pleased with my performance," Murphy, 33, said.
World number 30 Woollaston was amazed by some of the shots attempted by Murphy.
"He goes for balls that no-one else goes for. I don't think even Judd would have gone for some of the ones he went for," Woollaston said. "In the last frame I thought I played a great safety and he made a century.
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Murphy added: "Part of going for your shots is saying, 'I am prepared to go for my shots, are you?'
"It sends a message out to your opponent. I have always gone for my shots and I always will."
There were also wins for Englishmen Tom Ford and Martin Gould, Scotland's Stephen Maguire and Hong Kong's Marco Fu.
England's Judd Trump lost five frames in a row as he was beaten by China's Liang Wenbo on a dramatic evening at the UK Championship.