Order 66 was one of the most brutal arcs in all of Star Wars, triggering the slaughter of thousands of Jedi by their once-devoted soldiers, the Clone Troopers. Enacted by Chancellor Palpatine, the betrayal enabled him secret Sith Lord to tighten his grip on power, and rule the ascendant Galactic Empire, with Anakin Skywalker at his side as the newly christened Darth Vader.
Fans have frequently pointed to opportunities the Jedi missed to prevent the ambush from ever occurring, and, thus, save the Order and preserve the Republic. However, as one key scene in Attack of the Clones demonstrates, while there is plenty of blame to go around, Master Yoda bears much of the burden for missing the warning signs.
When Obi-Wan Kenobi tries to obtain information about Kamino from the Jedi Temple on Coruscant, he realizes the planet isn't in the archives. Even Jocasta Nu, the librarian, doesn't know where it is. Obi-Wan then meets with Yoda, who's training younglings, and as they scour the star system by hologram, one child points out someone may have deleted Kamino from the archives.
After dissecting this map for the wayward planet, it's clear Kamino's position is hidden where a gravity pull occurs. Yoda endorses the theory, and tells Obi-Wan to investigate the mystery in person. Yoda also admits the data must have been erased. But while he jokes about it being embarrassing, he should have taken the matter more seriously. Instead, Yoda meditates on the quandary, despite describing it as a "dangerous and disturbing" puzzle. He even admits a Jedi likely tampered with the archives. So one has to wonder why he didn't take the problem to the council.
For instance, working with Mace Windu might have revealed clues linking back to Yoda's former Padawan, Count Dooku, so the two could've stopped him in advance or gotten information from him. That Yoda also never kept track of Dooku, allowing him to become corrupted by Palpatine, says it all. It's irresponsible, and if Yoda did his job, he may have foreseen Anakin's dark turn too.
Being wary would have put him onto Palpatine's trail, so he might have actually been able to stop Vader from being born. The point is, Yoda's inaction and overall ineffectiveness feels like a rookie mistake. He's so amateur as he's caught with his guard down, which could have clued him into Order 66 and Palpatine's ambush with the clones.
To make it worse, he goes about business as usual in Revenge of the Sith, totally forgetting the spy and traitor in their midst. The fact that he knew something was off with Anakin should have made him monitor things even closer, but he can't reconcile this feeling with the archives being altered. This oversight makes it easier for the Clone Troopers to blindside the Jedi and render the them virtually extinct. Had Yoda acted proactively and searched for more details, the Jedi could have been ready, saving the galaxy from a period of oppression.