Ultima IX, as released, has a number of killer bugs that make the game unwinnable. The most notorious one occurred about two-thirds of the way into the game: some bad clipping code on a screen at the extreme edge of the game map caused some people to literally fall off the edge of the world with no way back into the game even though, visually, you were only one step away from being back on track. Worst of all, your saved games became invalid; you were forced to start over. Thanks to Executive Meddling and the general destruction of Origin by Electronic Arts, only three patches were released officially, and there were still game-breakers uncorrected by those patches.
Ultima IV omitted dialog entirely from a key character. The talking Horse in Iolo's barn was supposed to give you the answer to The Riddle. Instead, when you talked with it, it said "A". If you asked about "A", then it replied with "A" and asked you "A? (y/n)" If you replied yes, then it answered "A". Oops, default dialog! Unfortunately, this was the answer to the very last question you had to type and hit RETURN to win the game - the Codex's final question, of what Truth, Love, and Courage are made of. (Hint: it isn't literally "A.")
If you had taken proper notes, particularly during meditation sessions at each shrine (which were required), then the answer was fairly obvious. Still...
This became a running joke in the series. In each of Ultima V, VI and VII, Smith the Horse will say, "There was something I forgot to tell you" and give you a vital clue - to a puzzle from the previous game.
A more severe bug was present in the French version of the game. Because of how poorly the game was translated, the Text Parser recognizes neither the French nor English forms of items. Thus, you can't use items at all and the game cannot be finished.
In an effort to screw with pirates, Ultima IV has copyright protection that ramps up the difficulty of the game's combat to impossible levels if an attempt is detected to pirate the game. In the German version of the game this copy protection was activated on accident, making the game impossibly difficult and unplayable.
In Ultima VII, flying over the wrong mountain on the magic carpet would trigger a door lock in an area near the final showdown, which makes it impossible to complete the game.
In Ultima VII Part 2: Serpent Isle, a generic-looking bottle of "Serpentwyne" that can be picked up early on turns out to be critical to the plot. If you happened to use that bottle on one of your hungry party members, or if the game automatically chose it when you used the 'f' key, then you can't advance past the point where you needed it. The point where you will get stuck without it is most likely days after you picked it up. An identical bottle will not suffice, either.
On your way to the teleported mint, you pass a house with ghosts in it. The ghosts, unfortunately, have the dialogue of ghosts you meet much, much later in the game. Talking to them here can cause the game to become unwinnable.
In order to free the captain, you have to pay his captors a fine. If you offer them currency, they will increase the size of the fine to more than you have on you. There is a place where you can get gold bullion, but if you convert the gold into currency before talking to the guards, then they'll just up the fine again and you'll never get the captain out. This bug can also be a Game Breaker: The guards will take all the bullion you have on you as the fine. But if you drop all but one bar, you can free the captain, recover the rest of the gold, and convert it into enough cash to last you the rest of the game.