This reminds me of when I had a player take the keen mind feat and it was an absolute God-send for drip feeding lore. "Kat, you've seen this shape before, the bartender had a symbol like this on their ring"
I had to drop in a few irrelevant observations to balance it out, but it was great.
These sorts of Uno Reverse Card moments are both frustrating and gratifying to me as a DM. I of course try to roll with them, but occasionally they do mean I need to toss out half my mental notes for the rest of the campaign and seat-of-my-pants a whole new plot branch right in the moment.
There was one campaign I was in, I'd estimate it lasted about five years of real time, where my character stabbed the final Big Bad of the campaign with a weapon that we had picked up in the very first adventure of the campaign. We'd been toting it around ever since then without using it because it seemed like a very special purpose item. It wasn't pivotal to defeating her but it was still fun to tie the campaign together like that.
I think its fair that if your players do something that breaks your campaign in half, to say : guys, we can do this, but if we do then I have to redoe everything I have prepared. Would it be ok if we didn't please ?
But that is also why I rarely prep more than 3 sessions in advance. The more you have prepared, the more youll be tight with player freedom or loose more.
I've been a player for a few years and a dm for a few months now.
Hearing this as a player would completely kill my immersion and turn the campaign into an arcade game in my head. If you are allowed to say this, then I am allowed to hot reload when doing something stupid because having my pc die would throw out all of the work I've put into them and their back story.