DM's what has been your biggest "oh bugger" moment?

I'm talking when you had a series of things which you thought were unrelated and then they wind up coming together to cause you a real headache.

I made a giant forest boar as a boss fight for my table, which was bloody tough. The whole thing was set up like a Kaiju or a Colossus from Shadow of the Colossus, and has a lot of resistance to damage unless attacked in a specific way. It was also rather large:

 (the mini for the Monk is on top. Yep, it's huge!)

And it's also a CR15... Beast. And the Warlock just picked up Polymorph. And they're level 13, so I have 2 levels to work out how I deal with the shadar-kai Warlock transforming into a 40-foot high siege monster!

What's your "oh bugger" moment?