Yoda couldn’t sense Palpatine was Sidious
In The Phantom Menace the Jedi learn that the Sith had returned. During the Battle of Naboo, Obi-Wan and Qui-Gon Jinn fatefully cross sabers with Darth Maul. This battle ends up killing Qui-Gon and apparently Maul, then the Jedi ponder what Maul’s role was. They say there were only two, a master and apprentice but can’t figure out which Maul was.
In the film, we see that Yoda works in close proximity to Senator Palpatine. They are in many meetings together and brush shoulders on at least a weekly basis. Despite all this, Yoda, as powerful as he is, can’t sense that the Senator and later, newly appointed Chancellor, is in fact an evil Dark Lord of the Sith.
If Yoda truly was as powerful as he appeared to be, he should’ve been able to sense the darkness in Palpatine. Whether it was some counter Force powers on Sidious’s behalf or just ignorance and hubris on Yoda’s part, the fact he couldn’t realize this dark presence was controlling the war, was truly an unforgivable sin.