Playing as Genoa in EU3; In Domine, my first game, I did a lot of horrible things
1: I had a clear mindset of conquering and uniting Romania (I actually chose Genoa thinking it was Romanian country. This was back when Genoa had a province in Bessarabia). I was able to conquer Wallachia, but things didn't go to well from there...
2: I decided to convert to Protestantism when the reformation started while I had no protestant provinces (Oh, free money! Why am I down to -3 stability?) I lost most of my 'empire' because of that. I later was forced to convert back to Catholicism, another stability drop.
3: I had no idea I could adjust the 'treasury' slider that was there back then that could add money straight to my treasury
3a: as a result, I went bankrupt. another stability drop
4: After going bankrupt, my treasury slider was set to max, so I was making ALL the money! But also getting ALL the inflation, which I didn't realize existed until I was at, like, 100 inflation.
5: I don't remember how, but I ended up in a war with France
5a: as a result, I became a vassal. Consider me lucky
6: This is the one that literally ended my game: I didn't realize I could choose "no" on the demand vassal annexation with little consequence. (back when it was just a yes/no "Hey, do you want to be fully integrated?")
I'm sure I was also caught up in the weird alliance system that was present in EU3 and early EU4