Transformation of the wrong "kind" (Arrancar!Ichigo)
During the first battle against Fishbone D, Rukia prepares to transfer half of her power to Ichigo. Though everything happens pretty much the same, thanks to the Hogyoku inside her body, something unpredictable goes on: instead of having her reiryoku awaken his Shinigami powers, it goes to the Hollow part inside his soul instead, feeding it and having it prematurely undergo Shinigamification.
As soon as the Hollow attacks, Ichigo ends up outdoing it out of a desire to take it down a notch by being the bigger monster (figuratively) and a newfound hunger for energy (literally), wielding a sword and going straight for the kill... much to Rukia's horror noticing he ALSO has a hole in the chest. As he recovers, he assumes it's what she meant by giving him her powers, and panics when he sees her faint.
After a very guilty and curious Urahara gives her a gigai, she's told that if she doesn't want the rest of Soul Society to come screaming to kill this human that for some reason underwent Hollowification, they have to work on feeding him with any Hollow coming their way. And for that matter, Rukia isn't sure how to explain Ichigo's new form to him without making him freak out about becoming a monster (he'd eventually get over it so long he can still do good, but it also requires him not to use up too much energy in combat).
He also can't send souls to Soul Society, so Ichigo has to help certain spirits rest the old-fashioned way by comforting them and helping them handle their unfinished business, lest he wants his town to become infested with Hollows left and right every day (and attract unwanted attention from the Gotei 13 before the fact).
For a couple of days, he's initially believed to just be an anomaly, until they're given the address of the Visored to help him manage his current state. In the meantime, somewhere in Soul Society, Aizen wonders how and why the hell Urahara evolved Isshin's kid into an Arrancar already, with Gin joking the other scientist is trying to steal his thunder and make him look like a copycat.
It does make incriminating Rukia easier though, using accusations of all the possible risks she's running by believing she can tame a Hollow with Shinigami powers (and not so subtly hinting at some of the very things he does or plans to do with the Espada). Because Ichigo ends up being the first Arrancar created with the Hogyoku, he gains a slight nickname from humors in Hueco Mundo as the "wayward Espada", believed to either be an Arrancar abomination or one of Aizen's escaped experiments rebelling against him as he is on par with some of the Números.