City failed to capitalise on Cardiff's earlier slip-up against Ipswich and were beaten for the first time in 10 games, Wednesday running out 3-1 winners.
A miserable night was compounded by a missed penalty in stoppage time and a head injury suffered by goalkeeper Eldin Jakupovic in conceding his side's second goal.
Bruce's side remain second, but he said: "We probably made more mistakes today in 90 minutes than we have in the past three months.
"There were some crazy decisions by the officials. The linesmen and referee arguably had as bad a night as we had. For their first goal, Miguel Llera was a yard offside.
"But overall I can't complain because we've made elementary mistakes. We made far too many mistakes in terms of possession. We've made mistakes for all of the goals really, and we've been punished. It was a frustrating evening.
"We've been on a hell of a good run but we've got our backsides kicked. We'll have to bounce back."
Jakupovic collided with Reda Johnson a split-second after dropping the ball into his own net and was taken to hospital as a precaution.
Bruce said: "We hope he's okay. We won't know how he is until he's had a scan. We might have to use an emergency loan to bring in a goalkeeper."