random measures get a key signature, not just at the beginning of a system

I have deleted and added system breaks. Now I have a measure in the middle of a system that has a key signature, and I can't delete the key signature. I have actually a few random measures like that, that used to be at the beginning of a system and now aren't anymore, but still retain the key signature. I have turned off courtesy key signatures. I have tried deleting the measure. The only thing that I succeeded in doing was hiding the key signature on that measure. I want to get rid of it, and I want to know what i did wrong to create it in the first place. I'm on a Macbook Pro, with Sierra. Also, if you look at the score, the notes for the banjo are bigger than the notes for the violin and I don't know how to make them look the same.