Something often unnoticed about the famous testing-the-vaccing-on-himself moment for McCoy in 'Miri' is that Spock is the one who prepared the hypo.
At the beginning of the scene, we see Spock load it and set it down. Sure, McCoy checks it, looks it over, before injecting it. But ultimately, he doesn't change anything about it.
Something about trusting Spock's guesses more than other people's facts has me crying.











