Santino, in case you didn't notice was murdered as the result of his bad temper and despite Hagen's desperate attempt to stop him from leaving the compound.

Connie, after being widowed, whored around the world, lost her children and was reduced to being Michael's caregiver/substitute Don. After that opera she was alone in the world.

Mary despite warnings that her association with Vincent was dangerous was still arguing her point on the opera steps. If she had gone to the car quickly as instructed she would have lived.

Vincent lot the love of his life and shattered Michael by botching security at the opera.

I'd say those are fairly significant consequences.