After Dr. Lester, in Kirk's body, first arrives on the bridge and starts issuing orders, a stock wide shot of the bridge and viewing screen clearly shows Lt. Hadley, rather than Chekov, at the navigator station. At the tail end of this shot, Lester/Kirk specifically addresses his order to Chekov. In the very next shot, Chekov is suddenly at the navigator station to hear the rest of Lester/Kirk's order.
When Spock and McCoy are conversing in sickbay, McCoy is wearing his medical garb. However, in the first couple of close-ups, his shirt changes to his regular uniform.
In the closing credits, Lt. Galloway is misspelled, with one "l".
At 13:40 into "Turnabout Intruder" a blue blanket or coat can be seen hung over the back of the chair at the Navigation Subsystems station to the left of Mr. Spock.
At 14.10 Kirk/Lester gets up from the Captain's chair and says "Mr Checkov" but this is clearly an old scene as it's clearly not Checkov in the navigator's seat.
Lt. Galloway (David L. Ross) appears in this episode, despite being killed a year earlier in The Omega Glory (1968).
When Lester/Kirk is trying to get Spock to believe that she/he is currently a victim of the entity transfer, Spock replies that no entity transfer has ever taken place in the entire galaxy. Apparently, Spock himself forgot about having Henoch occupying his body in "Return To Tomorrow", or Sargon occupying Kirk's in that episode, too.
Kirk, in Lester's body, could make any number of references to the crew that Janice Lester would not know - for example, saying "Roger Korby" to Nurse Chapel.
Kirk/Lester asks Lt Lisa to play back the last two sentences of Spock's "tirade". However, she plays back only the last one.
When McCoy is in Kirk's quarters discussing Dr. Coleman, Kirk (being impersonated by Janice Lester) says, "Promotions and demotions can be politically maneuvered, you know that Bones." Although Lester wouldn't know Kirk called him Bones, this was possibly a slip on William Shatner's part as he's been calling him that for the past 78 episodes. However, she could have looked through Kirk's personal files and found the nickname.