When Mitch tells Carly that her wish that Spencer had been born normal has been granted, they are in the iCarly studio, which should no longer exist. This point is made later when the room is filled with junk and Mitch tells her there is no iCarly.
When Carly first enters the studio, the pear on Freddie's Pearbook is red. When Carly leaves the room, the Pearbook is blue.
After the fire, Carly storms upstairs into her studio and closes the door. After her first encounter with Mitch, she goes to leave the studio - and the door is open.