Jeremy Lamb scored 15 points, but the fourth-ranked Huskies blew a 17-point lead in the second half of a 68-63 loss to Central Florida in the semifinals of the Battle 4 Atlantis on Friday.
Connecticut finished with 13 turnovers in its first loss since March 5 against Notre Dame. The defending national champions had won 16 straight games.
Calhoun said it was one of the most "surreal" performances he has seen during his long tenure with the Huskies.
"I can't be more disappointed in how we played," he said. "We just stopped playing. We couldn't even get the ball inbounded near the end. Nobody stepped up and it was a complete disaster for us."
Marcus Jordan and Keith Clanton had 20 points apiece for the Knights (4-1), who had eight turnovers. Jordan also had seven rebounds and seven assists.
Connecticut put together a 10-0 run to take a 50-33 lead with 16 minutes left. Lamb sparked the run with a 3-pointer and also hit two free throws during the surge.
Central Florida then scored 17 of the next 19 points to pull within two. Clanton kicked off the rally with a 3-pointer and Jordan finished the spurt with a 3.
"That wasn't a run, it was an avalanche," Calhoun said.
Lamb made another 3 to stretch the Huskies' lead to five, but they couldn't hold off the pesky Knights. Jordan converted a layup, then made two big free throws to give Central Florida a 58-57 advantage with 3:11 left. Jordan added four more foul shots down the stretch to preserve the big win.
Jordan, the son of Hall of Famer Michael Jordan, said he received an encouraging message from one of his teammates before he made the two free throws that gave the Knights the lead.
"He told me this shot is in my blood," Jordan said.
Isaiah Sykes added 10 points for the Knights, including consecutive layups that stretched Central Florida's lead to 62-57 late in the game.
Andre Drummond finished with 13 points, nine rebounds and four blocked shots for the Huskies (5-1), who led 38-29 at halftime. Alex Oriakhi had 14 points and 10 rebounds.
"I feel like I let the team down," Lamb said. "I turned the ball over in key moments. We've lost games before but this hurts. We just need to come out with energy tomorrow and be ready to play."
Starting point guard Shabazz Napier finished with seven turnovers, and Lamb had three. Napier went 1 for 7 from the field and finished with four points and five assists.
Jordan was whistled for a technical with 16 minutes left in the game, fueling his big final surge.
"I still don't know why I got the technical," he said. "But I do know I wanted to come out and be more aggressive."
UConn now plays Florida State Saturday at 2 p.m. in the consolation game. The No. 20 Seminoles missed their first 15 shots and lost to Harvard 46-41.