Premature Empowerment

Some group of mysterious superpowered types gain their powers through a Super Empowering process that is permanent and irreversible. They want to induct a new member into their ranks—so they put him through that process with little or no explanation and then ask him to join them. What Could Possibly Go Wrong (Hint: Phlebotinum Rebel)

Anime and Manga

 * Often happens in Magical Girl shows, such as Sailor Moon (though technically, Usagi/Serena was born with the powers and only required them to be activated).
 * All of the senshi were born with their powers and awakened in a similar manner, though the series didn't seem to decide right away whether the reincarnated girls were "normal" girls who just happened to inherit some powers from their past selves, or were planetary warriors all along, reborn with their full power set (it eventually settled on the latter).
 * From Tokyo Mew Mew, all five girls had this (they were given their powers without being asked) and Ichigo's quest was to find the others after Ryou had injected them. Zakuro and Lettuce had the worst reactions to it, Zakuro just didn't want to join them (I think I remember something about her feeling like Ryou should've asked her first) and Lettuce's powers went out of control until she was told what was happening to her.

Film

 * In The Matrix, Morpheus feeds Neo a line about how nobody can explain the Matrix -- but doesn't exactly try very hard before giving Neo the choice of which pill to take. (Later, Cipher complains about Morpheus having done this when recruiting him, citing it as one of the reasons for his .)
 * Justified, as trying to do this when you're in the Matrix itself is one of those things that the Machines' search programs tend to pick up on (which generally means the Agents will be after you -- and before Neo became the One, you did NOT want to mess with the Agents). Besides, any explanation of the reality (or lack thereof) of the Matrix is a lot more believable once the person in question is actually out of the Matrix.
 * Parodied by Xkcd here.

Literature

 * Although no actual "empowerment" was involved, in The Hobbit Gandalf turned Bilbo Baggins into a dwarf-accompanying burglar practically before he had time to do the dishes.
 * Common practice in Urban Fantasy stories, most notably those dealing with werewolves, vampires, or other infectious paranormal. The Mercy Thompson and Kitty Norville books feature a number of characters to whom this happened, for example.

Live Action Television

 * Often happens in Kamen Rider shows, such as Kamen Rider Faiz (where the protagonist has the Transformation Trinket applied to him before he even knows what's going on), and Super Sentai as well.
 * It was once extremely common for Riders to be created by bad guys implanting kidnapped victims with technology, intending to make them their brainwashed warriors. You'd think they'd brainwash first and empower second, but they never do, and spend the rest of the season paying for it.
 * In Sanctuary, Nicola Tesla became a vampire with electrical powers after injecting himself with vampire blood (all true vampires having been killed off long ago). His goal throughout the series is to re-create the vampire race in his own image. His first attempt fails miserably, as the vampires are mindless savages with no free will. He then moves to Mexico and opens a rehab clinic for rich American kids, while covertly performing genetic experiments on them. Each of them is implanted with vampire DNA that is slowly changing their internal makeup and will not actually activate until decades later. Then one gets into a car accident and dies, triggering a premature transformation. He then proceeds to find and shoot every one who visited the clinic, doing the same to them. Unlike Tesla, they have no electrical powers. They then try to take over the world...Tesla does NOT approve.

Tabletop RPG

 * This often happened in Vampire: The Masquerade, with people being Embraced (turned into a vampire) without being asked first, then being taught how to act in their new lives.
 * It was a standard procedure of the Sabbat, which performed mass Embraces, with the victims being buried and those who dug themselves out being brought into the group.
 * The adventure "Blood Nativity" was about a group of people being Embraced simultaneously and being forced to fend for themselves without any training. Those who survived could join vampire society.

Web Comics

 * In the webcomic Panthera, the newest member of the group is asked to join the "Adventuring Club"…and then knocked out and injected with gene-altering treatments to turn her into a were-jaguar with elemental powers. And only then is she told what she's really let herself in for.

Web Original

 * In the Global Guardians PBEM Universe, The Blood Red King transformed Sister Mary Elizabeth Ross, a Carmelite nun, into Bloody Mary, a total psychopath with a taste for killing. The Blood Red King expected to get a servant. What he got was immediately attacked by his creation.