As one of the most beloved characters in the Star Trek franchise, Data represented what was best about its vision; exploration and discovery. Like Mr. Spock, his desire to be human through the adversity of not being one presented a way for viewers to examine the struggle for humanity from an outsiders perspective. As a crew member on Star Trek: The Next Generation, his abilities as a synthetic life-form helped the crew in incalculable ways, and they, in turn, assisted him in his quest to be like them.
For all Data's fantastic qualities, his pursuit of becoming human often got in the way of his duties, his friendships, and the safety of the crew. His inexperience with human emotions, as well as his overall naivete about human interaction was as much a source of danger as it was a source of humor. Some of the things Data did wrong were easily forgivable, but others took much longer to overlook. Here are ten of the worst things Data has ever done.
FAILED TO CHALLENGE HIS FATHER'S IDEAS
Data is the progeny of Dr. Noonian Soong, an innovative scientist that sought to create an android in his image. It desires to be human because he programmed it to. It has a modesty protocol because he desired it to be similar to humans in that way, even though Data has no need for clothing. It is the apex of egoism, and Data was never built with the capabilities to confront it.
That the universe would be better if everyone were more like us, is the sort of ethnocentric viewpoint that Captain Picard and the ranks of Starfleet would abhor. By not being able to tell his father, "I may look like you, but I'm not you", Data robbed himself of his ability to be fully Data instead of fully human.
NEVER CHALLENGED THE CREW'S OPINION OF HIM
The crew had a sort of fascination with Data's "Pinocchio Complex", often coddling him like a newborn baby as he took his first steps to become human. Because it was something he wanted to do (correction, was programmed to want to do), they tried to assist him, rather than ever stopping to think whether or not it was the right path. Data is child-like in the sense that he never questioned Dr. Soong's destiny for him, something all adolescents eventually do of their parents.
Dr. Soong was a human, Data was created in his image, therefore he must be human. It was only ever Captain Picard that stressed to Data that he could be more. Data could simply be worried about being the best android he could be and stand up for himself when the crew infantilize him or treat him with suspicion (all of Season 1 for Riker).
SUBJECTED EVERYONE TO HIS POETRY
On a lighthearted note, Data subjecting everyone to his poetry isn't inherently bad if you're a fan of Data, but on Star Trek: The Next Generation, it was a running joke that while the crew may think fondly of their android friend, they don't think that highly of his poetry. This may be because his poetry often has to do with subjects they aren't interested in, like extolling on the virtues of his cat in "Ode to Spot".
Riker regularly falls asleep during Data's poetry readings in Ten Forward, and the general malaise of the audience indicates they're simply waiting for it to be over. This is in sharp contrast to whenever Data is performing in the ship's quartet, which is a well-received occasion, and an opportunity for him to garner praise.
SCREWED UP HIS RELATIONSHIP WITH JENNA D'SORA
Besides his one night stand with Security Chief Tasha Yar, Data hasn't had that much experience with intimacy when it comes to women. That's why when fellow crew member Jenna D'Sora desired to pursue a relationship with him, it offered the perfect opportunity to gain some. The biggest hurdle of course was that Data has no emotions, and therefore will never fully be as invested in the relationship as D'Sora.
Data creates a romance program to run when he's seeing her, compiled of all the appropriate responses (to give her flowers, compliment her hair, ask about her day), but she describes it as hollow and forced and terminates their relationship. Data shouldn't have made a program, and he also should have dissuaded Jenna from pursuing him without fully understanding what it meant; they both know better.
MEDDLED IN O'BRIEN AND KEIKO'S RELATIONSHIP
Most of what Data does, he does out of a compunction to better assist his fellow crew members and his friends. While he may not be emotionally invested in them like a human, he "gets used to them", and appreciates seeing them happy as it means he has accomplished his goal in improving their existence just as they have improved his. Occasionally though, he makes epic blunders that cause more harm than good.
In "Data's Day", Transporter Chief O'Brien and his fiance Keiko are to be married on board the Enterprise, which inspires Data to learn how to dance for the ceremony. It also inspires him to stick his nose where it doesn't belong, by speaking separately with O'Brien and Keiko, his logic stirs up emotions that almost lead to them calling the whole thing off.
UTILIZED HIS EMOTION CHIP
In the episode "Brothers", Data gets the upgrade he's always wanted - an emotion chip from the very man who created him, Dr. Noonian Soong. You would think such a momentous occasion would be cause for celebration, but the Data that utilizes his emotion chip is annoying, hostile, and able to be very easily manipulated.
In Star Trek: Generations, when Data's emotions are in full effect, he's so consumed by the thought of getting hurt during an attack, that he cowers in a corner while his best friend Geordi gets taken hostage, which only causes him to have a further emotional meltdown. It would have been best if Data utilized the emotion chip incrementally until he understood its effects better.
CREATED OFFSPRING WITHOUT CONSULTING HIS CAPTAIN
To Data, it seemed only natural that one day he should attempt to create another being like himself if only to be less alone in the universe. After attending a cybernetic engineering conference, he decides to get to work right away on creating his own progeny, an offspring like himself, that he could teach things to and also learn things himself (after all, part of being human is procreating).
However, he doesn't get his captain's permission, which puts Captain Picard in a very awkward position with Starfleet Command. Naturally, Picard knows that Starfleet will want to take Data's offspring from him to study, and Picard knows that he will have to go toe-to-toe with Starfleet brass to prevent it from happening.
LIED TO HIS FELLOW CREW MEMBERS
In the episode "Clues", the Enterprise goes through a strange cloud of space vapors, and the entire crew passes out for thirty seconds. Data revives them, explaining that they went through a Black Hole, which he was not affected by because of his synthetic composition. They would be inclined to believe him, except that there are aspects of his story that don't add up.
Over the course of the episode, the crew realizes that an entire day has passed, not just thirty seconds. This means that Data lied to them. He continues to obstinately lie to them throughout the episode, until the very end, when it's made clear that he was forced to for circumstances involving the mysterious vapor cloud they passed through. He was only able to override his ethics protocol because he'd been given a direct order.
FOLLOWED HIS EVIL BROTHER LORE
Though Data often thought himself alone in the universe, it wasn't entirely true - he had a twin-brother also created by Dr. Soong named Lore. Lore believed there was more to life than being an android and a slave to humans, so he sought to be more. He was able to have this sort of superiority complex because he used the emotion chip that Dr. Soong also designed for Data.
The promise of the chip was too great for Data, who had always dreamed of experiencing his own emotions. He betrays the entire crew of the Enterprise, allowing them to be captured by Lore and his Borg cronies in "The Descent Part I", and even going so far as to perform dangerous surgery on his best friend Geordi via his implants.
DATA HOLDING OUT AFTER TASHA-YAR
Though an alien anomaly led to their one and only act of intimacy in "The Naked Now", Chief of Security Tasha Yar and Data participated in one of the most bond-forming rituals humans can perform. Not only have Star Trek fans speculated how an android could be "fully functional", but they've also speculated why ever since the incident.
Why did Dr. Soong make Data have these capabilities, if not to use them more than once and not just when aliens are involved? He gets an actual girlfriend Jenna D'Sora, who's unaffected by extraterrestrial trickery, and Data doesn't see her as any more deserving of that sort of affection than his one-night stand. He should have listened to Tasha Yar when she told him to forget the whole thing ever happened.