Besides obvious 'send exploring fleet and lose it instantly in the middle of the ocean' I recently had particularly idiotic mistake 
Ottomans, Ironman, first ever Muslim + first ever Ironman + first ever AOW game, I am conquering everything nice around 1470, I reduced Byzantium to OPM Greek island, finally I decide to invade it.
Forgot about truce.
I have absolutely no idea how I managed to do that despite warnings and actually declare war and only THEN notice -5 stability hit and giant coalition forming against me, it was even more idiotic because it was first time ever I did that and obviously in Ironman.
Result:
between 1470 and 1500 I had chain of 3 huge wars against - in various combinations and coalitions - Austria, Venice, Bosnia, Wallachia, Ragusa, Cyprus, Mamluks, Aq Qounlyu, Ramazan, Qara Qounlyu, Georgia, Circassia, Karaman, Fezzan, Castille, Aragon, Naples, Tuscany, Papal State, Switzerland, Hungary, Wurzburg, Saxony, Brandenburg, Genoa, Haasa, Hansa, Munster and few other minor countries.
The final result was nice as I emerged as major world power during these wars, but in the first one there was a moment half of my European land was occupied and I was close to ragequit
What about you? :ninja:
Ottomans, Ironman, first ever Muslim + first ever Ironman + first ever AOW game, I am conquering everything nice around 1470, I reduced Byzantium to OPM Greek island, finally I decide to invade it.
Forgot about truce.
I have absolutely no idea how I managed to do that despite warnings and actually declare war and only THEN notice -5 stability hit and giant coalition forming against me, it was even more idiotic because it was first time ever I did that and obviously in Ironman.
Result:
between 1470 and 1500 I had chain of 3 huge wars against - in various combinations and coalitions - Austria, Venice, Bosnia, Wallachia, Ragusa, Cyprus, Mamluks, Aq Qounlyu, Ramazan, Qara Qounlyu, Georgia, Circassia, Karaman, Fezzan, Castille, Aragon, Naples, Tuscany, Papal State, Switzerland, Hungary, Wurzburg, Saxony, Brandenburg, Genoa, Haasa, Hansa, Munster and few other minor countries.
The final result was nice as I emerged as major world power during these wars, but in the first one there was a moment half of my European land was occupied and I was close to ragequit
What about you? :ninja: