It's fair to say that the "Buffy the Vampire Slayer" pilot, "Welcome to the Hellmouth," is one of the better ones, as it explodes out of the blocks and sets the tone for what's to come. However, it wasn't what was originally shot when creator Joss Whedon pitched the show known only then as "Slayer" to The WB.

According to the biography "Joss Whedon: Geek King of the Universe" by Amy Pascale, Whedon produced a compact 25-minute pilot. While several storyline elements from what would eventually be broadcast stay intact here, a few of the important characters that appear in the aired pilot are missing, including David Boreanaz's Angel (one of the best characters in "Buffy the Vampire Slayer") and Kristine Sutherland's Joyce Summers. In addition, there are different people playing Principal Flutie and Willow Rosenberg: Stephen Tobolowsky and Riff Regan, respectively.




The WB wasn't impressed with the pilot, nor was Whedon who vowed to never release the pilot publicly (it found its way onto the Internet, though, since that's a law unto itself).

Initially, the network passed on commissioning the series, but some of the execs were intrigued by the premise and thought it showed promise. So, when "Savannah" was axed, The WB invited Whedon to have another stab at "Buffy the Vampire Slayer" to fill the slot. Whedon tweaked and expanded upon elements of the pilot, which included recasting Tobolowsky and Regan with Ken Lerner and Alyson Hannigan, and the rest is TV history.

https://www.tvline.com/2016387/tv-show-pilots-reshot-before-airing/