Early Edition had an episode that focused on a girl with psychic powers.
Big Time Rush premiered a two-part pilot episode (Big Time Audition) as a "sneak preview" on November 28, 2009, but the series itself debuted the following year on January 18.
Averted on Prison Break. The producers planned a spin-off with the working title Prison Break: Cherry Hill, which was to focus on a woman escaping from a maximum security prison. The main character was to be introduced in an episode of its parent show, but first it became too difficult to steer its serpentine plot in a direction that could accommodate ''Cherry Hill", then casting the lead became a chore, then finally the writers' strike made it more trouble than it was worth.
Spoofed in NTSF:SD:SUV:: with the "Time Angels" episode, which sees NTSF:SD:SUV agents team up with a group of hot time-traveling crime fighters who basically do everything and overshadow the regular cast. As Trent Hauser says at the end of the episode, "If they had weekly adventures I could watch or DVR, I would do it in a heartbeat." During credits, there's a fake ad for the Time Angels spinoff. Note that, since NTSF:SD:SUV:: itself spun off from Childrens Hospital after a one-off gag appearance, a Time Angels show isn't out of the question. In season three the gag got even more blatant, with an entire episode dedicated to the Time Angels wherein only two regular characters show up, and even then only in the Framing Device..
Pruned these when I launched the subpage and alphabetized. They either need more context or aren't examples.