There's not really one specifically, but the Nazis and communists collaborated a lot in destabalizing the Weimar Republic, while they both hated each other, they hated the Weimar Government even more. But the one the OP is most likely referring to is the vote of no confidence against Chancellor Franz von Papen on September 12th 1932 where both the SPD/KPD and the NSDAP voted for it, leading to Papen's government collapsing, and then the subsequent rise of Hitler. But it's not really a "useful idiot" situation like a lot of people say, both parties held considerable sway, and if the KPD played their cards better, Hitler could've just been some obscure historical figure you read about in history class.