Like exactly what unreal elements you introduced. If the sky is always, always, always overcast, you can't have any sunlight glancing from the waves of the sea.
At any time.
Not just when it's plot significant that it can't.
It's a problem that many writers have. From letting your characters frolic along a tunnel you have neglected to provide a lighting source for, to having characters at one point carefully distinguish between addressing a woman by her first name, and by her last with a title, while everywhere else everyone calls her by her first name with even less excuse that the characters who distinguish.
I suspect it's because you have to remember all the time.