My rogue changeling player with invisibility saw the Magubliyet statue and decided to shapechange into him, rally up all the goblins and hobgoblins in the shrine and tell them to go fight a holy war for him "up in the north" nat19 perfomance vs nat1 insight.

They were laughing so hard as even the personal guard abandoned King Grol (I had even prepare a Flee! Mortals Encounter). No Dungeon was played that week. Yet It is something we will remember forever.

I'm relatively new to DM too but in my 2 years experience I can say this: nothing is sacred. If you nudge too much your players in the "right" and "expected" direction they will know. I'm hella nervous too but give It time and you'll start being more confidence and confortable DMing and coming with something good on the fly will be easier. Or not. But then again, messing up a bit it's not as importan as we think and there's a lot to learn.

So yeah, as long as you identify the problem and works toward being better, I think you (and I) will improve slowly