"We were dreadful from the first minute and it's the most disappointing performance of the season," said Cumbrians boss Paul Simpson. "Right through the team we just turned up and expected the result to follow.
"It would be unfair to blame one mistake by Matt Glennon for the result, the whole team played badly.
"We have let ourselves and the supporters down but we have got a chance to turn it around on Tuesday against York City," said the Blues boss.
Glennon's goalkeeping gaffe gifted relegation-threatened Forest Green Rovers their first three points since Boxing Day.
A poor clearance by the keeper was tidied up by David Beharall at the expense of a corner and the United defence dealt comfortably with the cross, but Paul Warhurst pumped the ball back into the box and Glennon dropped the ball and former Carlisle defender Damon Searle side-footed it into the bottom corner.
It was Rovers only shot at goal in the whole game and their victory owed much to a solid defensive performance by the experienced and much travelled Searle, Warhurst and Des Lyttle.
Goal scorer Searle said: "I don't get many goals but I am pleased with this one as it was important for us today."
United were expected to win and Karl Hawley gave the visitors a scare with a shot that beat Dean Williams, but ended up in the sidenetting.
Adam Murray then went close with a low shot that flew just wide from the edge of the area and Williams did well to tip Peter Murphy's free-kick over the bar.
A Warhurst error gave Derek Holmes a sight of goal, but he blazed over the top from 20 yards before Williams made an easy save from a Hawley effort from an acute angle.
Simpson brought on all three substitutes in an effort to find the equaliser but it was to no avail.