Overall, I'd say that Captain Kirk was a very effective, successful captain. He and most of his crew survived, with the exceptions of the Enterprise B, where he was sucked into the Nexus. But he did make some mistakes along the way. Some more grave than others. In TOS: From what I've seen, I'd say that one of his worst was in the Immunity Syndrome, where he took the Enterprise into the "zone of darkness." A probe was launched to investigate and they lost it. Vanished. No trace. They could have even programmed a probe to penetrate and then immediately reverse course to return... and they'd have found nothing coming back. So... taking the Enterprise into it was an enormous risk. For all they knew, it could have been "space quicksand," with no means of getting out. Or it could have been some manifestation surrounding a black hole, where entry would mean being drawn into it, never to escape. Given what was at hand, what was known about the phenomenon, I think Kirk made a huge mistake going into the zone. It would have been far safer to have one or more additional ships in the area attempting to deal with this phenomenon. Perhaps they could have come up with a specially prepared probe with greater power and attempt a partial penetration to see if they could at least test the limits of the barrier. Had the Enterprise not entered the zone and waited for a couple more ships to assist, it is possible that the space organism might have divided and become two. But the method of destruction devised in the episode showed that they could have applied it to both organisms. Kirk and his crew got lucky... very lucky, with Spock's major risk to explore the organism (discovering that it's ready to reproduce and suggesting they destroy it) and McCoy's idea about anti-bodies. And Scotty's engineering fortitude keeping the Enterprise intact. In Bread and Circuses, I was also quite amazed that Kirk took his risk of heading into the city in search of Captain Merik without much more cautious surveillance. They got caught pretty easily, when they had tricorders capable of detecting life forms from miles away. They could have also communicated with the Enterprise to scan the surface and give them a better path to at least investigate possible ways to assess the city perimeter. Given when took place after being captured, they could have all ended up dead by that grave mistake. Another mistake I'd say Kirk made was with Lazarus, in The Alternative Factor. Far too trusting of the guy. Had he been under closer surveillance, they'd have figured out his agenda much sooner. In the Movies: He was far too trusting of the Reliant's silent approach in The Wrath of Khan. What an absolutely dreadful error. And it not only cost many lives, but also caused the death of Mr. Spock. You'd have thought at that point, Kirk would've been smarter... and even Spock, who would've been less punitive with Saavik and have suggested shields be raised. Any other critical mistakes you think Kirk made during his command?