First of all, Al Taamin is not necessarily about Tzadikim. Abaye holds that a Tzadik will not go off. The Gemara we had recently says that if most of his life he was a Tzadik, Hashem will make sure it stays that way. Yishmor Raglei Chasidav. The Tosafos Yeshanim reconciles this with Yochanan Cohen Gadol that although Hashem puts things in place to almost force him not to sin, he can overcome them.
The Maharal explains that this is what happened with David Hamelech. As Chazal explain, it turned out not to be a Chet. But since, according to his actions it would have been, it was held against him. Ayin Sham.
If someone overcomes Hashem’s messages and roadblocks it is because he is heading in a certain direction. Now, not only is this example very uncommon, that we always have to point back to Yochanan Cohen Gadol, it is also never used to say that Tzadikim are Chote ChV from time to time. Remember the Gemara in Brachos 19 about someone who spoke about a Tzadik.