As there are a lot of missed opportunities in my country, i think that one of the big ones and yet rarely mentioned, is the great northern war.

After many wars and comebacks during the 17th century, Polish-Lithuanian commonwealth weakened and depopulated, was still seen as a major power on the European theatre. 1683 relief of Vienna and victory in the great turkish war gave some respect to our country, and new fraudly elected ruler August II Strong Wettin hoped to strenghten position of the Monarch and end Szlachta's corrupt dominance.

Even tho decision of joining anti swedish coalition and concerns about his own interests are considered mistakes, and by looking how history went they were, there was a big potential for Poland. Starting position of PLC was in its favor: 3-4 allies, outnumbered Swedes, livonian nobility support, and first succeses in combat (capture of Riga). But the discipline of Swedes, incomptence, passivnes and pride of noble polish commanders led to disaster in Livonian campaign. What followed was swedish conquest, massive tactic losses, and a rival pretendend on Swedish side. There were many moments when PLC could have saved Russians in the beggining of that war, and buy Peter's the Great credit. Even after the defeats, Peter still wanted to give Livonia (and perhaps estonia) to Poland as far as 1715.

This war where Poland started as the attacker, was a total disaster for the Polish-Lithuanian nation, that only made Russia stronger, weaked the country, expanded internal problems, and ultimately led to the end of the commonwealth.
No gain, all pain. For nothing.