PSG coach Laurent Blanc said wasting a two-goal lead against was "inexplicable."
Blanc's men led 2-0 after 20 minutes through Lucas Moura and Adrien Rabiot, but Bastia's Ryad Boudebouz scored a penalty in the 32nd minute, and the French champions eventually fell to a 4-2 defeat and missed a chance to move to the top of the Ligue 1 table.
Blanc said PSG's own complacency was to blame for the setback.
"Yes, we were complacent at times. The three points were within our reach," Blanc said. "You get taught a good lesson when you don't do things properly.
"It's a big mistake from us; we woke Bastia and their fans up. We have no right to do that. Conceding four goals is inexplicable.
"I saw things that I do not like. We did not do what it takes to win."
A two-goal victory would have seen PSG leapfrog Lyon and Marseille and claim top spot in Ligue 1 for the first time this season, but it was not to be.
The coach cautioned that Saturday's disappointment should not be wholly attributed to a mental lapse.
"The future will tell whether it was an accident or a relapse in our state of mind. Don't pin everything on our mental state," Blanc said.
Star striker Zlatan Ibrahimovic, who is still struggling for fitness since returning from a troublesome heel injury and has scored only once in his past four games, was stunned by the defeat.
"We played well for the first 20 minutes and then something happened which has never happened to me before," the burly forward said. "I mean, winning 2-0 and losing 4-2, I've never experienced that. We need to wake up."
Information from The Associated Press was used in this report.