The most simple example is if your lv1 char meets some lv99 guards
Just to be clear on this as well - in a properly made RPG you won't find level 99 guards. In single player RPGs you/mobs aren't powerful because you/they have levels, you/they have levels because you/they are powerful. Witcher 3 is absurdly bad in this. Randomly stumbling on 30th lvl bandits that are tougher than mythical beasts you fought 5 levels ago. High level trash mobs are MMO design and it works good there, but is nonsensical in single player RPGs.
In another discussion (at obsidian? I am not sure) somebody gives an example.
That's probably the Tortured Hearts module for NWN. A Dance with Rogues may also have such a sequence.
I dislike that you cannot fail the main quest and that people drop a note when you kill them.
I think Swen may have missed the point on this. The main argument for the main (and all other) quest/s in RPGs is that it can be completed in multiple ways, NOT that it can't be failed. However that may simply be his design choice, though I can't understanding the reasoning behind it.