When a TV show is pitched to a network, the creators behind it put together a pilot episode to give studio executives a taste of what they're peddling. In some cases, the sampler is well-made enough to air with minimal changes and goes on to serve as the debut episode of the commissioned programme.
More often, the broadcaster draws up a list of alterations that must be in place before the green light is possible, and these can range from minor tweaks to dramatic casting changes and tonal shifts. In other words, episode one ends up nothing like the pilot.
Lacking studio polish, most TV pilots are rough around the edges and not intended for public consumption, though many of them end up leaking online or wind up among the DVD extras section when the boxset is released.
Looking back at pilot episodes of your favourite show can be a weird experience, almost like glimpsing into a parallel universe. Certain characters act differently, others looking nothing like the version you know and love, and some are absent entirely.
Here are some of the hit television shows that made the most sweeping changes when making the jump from pilot to full season.