Real Life Writes the Plot — The reason why the Avatar opens the Stygian Abyss by summoning Pyros, rather than the originally scripted Slasher of Veils. Because of the development schedule being cut, and the fact that they'd already created a CGI Pyros for an unrelated cutscene in the original plotline, it worked out much quicker to just stick him in the cutscene where the Slasher would have been summoned, and alter the dialogue accordingly.
For those who don't know why this is bad... Pyros is not supposed to exist anymore. He's gone, and his essence was consumed by the Avatar back in Ultima VIII.
Even worse- in the game "Ultima Underworld: The Stygian Abyss," you actually destroy the abyss.
The Other Marty — Michael Dorn originally recorded the Guardian's dialogue for this game. Due to the amount of Executive Meddling however, the storyline ended up being radically altered, and Dorn was too busy with Star Trek: Deep Space Nine to record the new dialogue. As a result they ended up having to bring back the Guardian's original voice actor, Bill Johnson.
What Could Have Been — There are screenshots in existence of the early build of the game, which would have been viewed from above (like the other games in the series), but still featured environments and characters which were rendered in 3D. The decision to rework the entire game engine into effectively being a third person shooter was made by the game publishers because 3D games were popular.
Just check out this website, which details the rather exaustive development process.