”You can always count on a Vulcan’s inability to lie.” Vulcans lie all the time.

Especially fun in the context of Star Trek VI:

SPOCK: Curious. Very well. In twenty-four hours we'll agree this conversation did not take place.
VALERIS: A lie?
SPOCK: An omission.

SPOCK: Thank you, Mister Scott. Lieutenant, please inform Starfleet Command our warp drive is inoperative.
VALERIS: A lie?
SPOCK: An error.

KIRK: Names, Lieutenant.
VALERIS: I do not remember.
SPOCK: A lie?
VALERIS: A choice.

Spock is somewhat easy to justify, being half human. Valeris is trickier. Had she remained Saavik, and been revealed to be half-Romulan as planned, it would once again have been easy to justify. The change from Saavik to Valeris, and dropping of the half-Romulan aspect, means we have a TOS-era Vulcan following in the "it's not a lie if I can use a different synonym" pattern Spock uses, so more or less renders the whole thing untrue.