There were two instances in The Next Generation where Data can be said to have lied or at least severely distorted the truth that I can remember:
- "The Most Toys" (Season 3 Episode 22) - Data misrepresents how a weapon was discharged.
- "Clues" (Season 4 Episode 14) - Data is ordered by Picard to lie to cover up the existence of an alien planet.
In "The Most Toys," Data was going to kill his captor and was transported off the ship as he pulled the trigger. When Riker tells him the weapon was in a state of discharge, Data says "Perhaps something occurred during transport, commander." While this might not technically be considered a lie as he was not asked a direct question, it is bending the truth right up to that point.
Other than the examples listed above, are there any instances where Data explicitly lied without being ordered to do so?