Pam B I suppose it depends
I suppose it depends on what the plug is made of…. I doubt cork would work LOL. Just kidding… My larger kiln has a hole with a movable cover on the side so I can vent the kiln when I want. I usually open it when the major part of the annealing process is done and it is just cooling down. It is probably just a vent hole not a defect but it will need to be closed during most of the fusing process.