(Building Encounter rules for reference: https://2e.aonprd.com/Rules.aspx?ID=497)

Story time: Our group went down into a dungeon, slightly too far without clearing everything above. We were doing rather well, having a very balanced party. Now at some point we went back to town and when we came back we decided to check on the part of the upper levels that we missed. Since the party level was already one level above the intended encounters, the GM decided to put together multiple of the "lower level" encounters to make it a bit more challenging ... however it turns out he exceeded the extreme encounter budged by doing so by more than one quarter, which invariably led to a TPK after a hard-fought battle with full resources used from the party's side. We did have a decent tactical advantage, otherwise it would've been even more of a massacre. I can only imagine if we didn't have 100% of our spell slots.

Moral of the story: GMs please be careful when you put encounters together. Recalculate the encounter budget and bear in mind, that - according to the rules as written and intended - you should only put extreme encounters in front of your players very sparingly. Exceeding the encounter budget by quite a bit will likely prompt a TPK, especially if the party is not fully rested.

Also please be very careful with monsters that are more than 2 level above the party, those can often be harder than expected (especially if they have already have a high AC for their level) and might also be rather frustrating to fight against for the players.

Thanks for listening to my TED talk rant.

TL;DR: read and follow the encounter building guidelines, they are actually accurate and the encounter budget works!