I am not sure why this keeps happening. about 50% of the time when I design a model for a reactor and make a render of it, the top plane shows up even though it is actually hidden. I've rendered a bunch of different shapes and models but never had this problem. It only seems to be the reactors and only sometimes. It's happened 3-4 times of the 7 or so I've made. Every time I end up with this problem I end up scrapping the file and starting over.

Any ideas why this happens or how to fix it? Any help would be greatly appreciated.

(the top plane is disabled visually)