I know of a few pilots (one of whom I know well, and he's a smart dude who prepped hard) who have failed the cog and a couple others who have failed on knowledge.

IMO, the cog test is a less-controllable variable. The one guy I know well who failed the cog test thinks he knows exactly when it went downhill - he just got flustered and wasn't able to recover. He'll pass next time, I'm sure, but 6 months of seniority is 500 numbers! That's going to take a lot of banana peels to recover from.

The knowledge test is something you can prep for much more quantitatively. There are definitely "discriminator" questions out there, some of which I'm convinced are placed to be time sucks, to give you the opportunity to make the poor decision to spend 5 minutes on that question instead of skipping it and driving on. Yet though just about everybody studies their posteriors off, everybody also seems to walk out thinking there will be a semi-canine offspring around a few months later who looks vaguely like the guy they saw that morning in the mirror shaving.

Prep or stay home. Say what you want to about the interview process, but if nothing else it probably does a good job of screening out the lazy folk.