The Star Trek Compendium is a great reference for show info, bloopers, etc., but here is one they didn't catch. Credit actually goes to my brother's wife who noticed this one. Good eyes, Sis, but why exactly were you staring THERE?!?
In episode 46 "The Gamesters of Triskelion" (aka The Pulsating Gummi Brains), we get to see exactly how tight fitting Captain Kirk's pants really are ...
1. Forward to final fight sequence (about 4/5 of the way through). Kirk is fighting 3 opponents by himself.
2. Kirk kills the first, causes the death of the second, and is left in a knife fight with the remaining opponent.
3. The blue alien (an Andorian?) charges Kirk. Kirk, back to camera, ducks to avoid knife stab.
4. As Kirk picks up Andorian to flip him over, camera cuts to wide angle shot. Freeze here immediately and look in the general direction of Kirk's, er, naughty bits.
5. Thank the Lord that Shatner was wearing something.