Not the worst line, but one that always bugs me: When the Fesarius pulls up in front of the Enterprise and Spock says "Must be a mile in diameter!"
Given that the Enterprise is almost 1,000 feet long, a mile-diameter Fesarius would be a tad over 5 1/4 times the length of the E. The effect shows it to be a LOT bigger than that.
Also, for a guy who regularly estimates things down to 3 decimal places, "must be a mile..." is pretty sloppy.
To be fair, Spock in Corbomite Maneuver is a character still in beta-test, only two episodes removed from the emotional Spock of "The Cage". It took a few more episodes for the 3 d.p. Spock that we all know and love to emerge. Also, perhaps the director didn't know what the Fesarius was even going to look like, or the guys that did the f/x didn't know that it was only supposed to be a mile or so in diameter when they created the model footage.