Encounter at Farpoint (1) - S1-E1
Character mistake: In his exchange with Admiral McCoy, Data uses several contractions (i.e., shouldn't, I'm). For the rest of the series he doesn't, and in S3: Ep16, "The Offspring", he admits that his programming makes him incapable of using contractions. And in S4: Ep8, "Future Imperfect" Riker questions Data on when he started using contractions, knowing that Data is incapable of it.
Suggested correction: It is true that in later seasons he is unable to use contractions. However, since that characteristic was not introduced in the first season, this is not a mistake. His behavior is consistent with the character's behavior throughout the first season.
It's still a possible mistake, even if it wasn't part of season 1 scripting. Just because we only learn later that Data doesn't use contractions, doesn't make this mistake invalid. We learn Data was specifically constructed that way and hadn't been able to use contraction since being created, which means all of season 1 he shouldn't be using contractions. Although it would probably be best to submit it as a mistake in season 2 when it's mentioned that it contradicts what was established.
Bishop73
Incorrect. It would be a mistake if he used a contraction after the trait was added to his character, but the mistake can't be retroactive as it wasn't a mistake for him to use it at that point in the show.