Just before Lenin's death during the 12th Party Congress, Lenin denounced Stalin pretty harshly and called him rude, disloyal, selfish, ambitious, and recommended his immediate dismissal. When Lenin died not long after, Stalin was accused of poisoning him and chased out of the USSR.
But what if Lenin had not been so harsh in his letter? Could Stalin have come to power? And what would his Soviet Union would've looked like, if he was as nasty as Lenin claimed?