My first D&D experience was when I was 12, and I was our group's DM. (Back in the days of 2e) I spent forever designing this intricate island maze with the McGuffin at the center, that I thought would take at least two sessions to get through. Anyway, the party had just hit level 5 the session before and the new session opened with them landing on this island.
Me:
reading a detailed description of the maze entranceMage: hey, do I see any creatures flying above the maze?
Me: Um, no.
Wizard: Cool, I cast
fly on myself and go over the wall
Me: Wait, what? You can fly?
Wizard: Yeah, now do I see the open altar where this thing is supposed to be at?
Me: ...ah, shit.
And that's how my party learned how to fly^^ Now of course, I could think of dozens of ways to handle it, even if I had messed up that bad in the first place
But back then all I could think about was my hours and hours of work going to waste. It did teach me an important lesson about keeping an eye on the party's capabilities though^^