Although King's Quest VII is the only game in the series to avert Unwinnable by Design, there is one bit that breaks the game on newer, faster computers. At one point, you have to pick up a lit firecracker and quickly put it to use before it explodes right in your pocket. However, being designed for slower machines, on a modern computer it may explode near instantaneously.
However, you respawn in the spot you died, so you can still make it if you just keep walking between deaths as can be seen in this video.
Due to a spot of bad programming, a timer at the end of Space Quest IV runs based on hardware speed. Nowadays, it can take less than a second.
So is the zombie/robot timer at the beginning, in some versions. On modern computers, either shows up the moment you arrive where they can show up.
An antidote is available for computer-savvy hardware-speed timers: DOSBox. Just use this with an appropriately low number:
Space Quest VI has the infamous "Error 47" bug, which freezes the game when using the ComPost in sickbay. This was eventually traced to a timer problem, but unlike the other instances, many computers available at the time of release had this problem. It wasn't fixed until fan-made patches were released several years later.
Gold Rush has a timer problem opposite of the usual: The faster your computer, the slower a hidden timer runs. It means that even on a computer as slow as 1998 speeds, the realtor at the beginning never shows up in any reasonable amount of time. (Setting the game speed to "slowest" makes the timer run faster, so it's passable if you know about this glitch.)
Quest for Glory
Quest for Glory IV has an insidious bug which causes the game to crash on faster computers when the player enters the screen with the Chernovy. This makes it impossible to fight the Chernovy, which you must do to beat the game. (There are third-party solutions, like running the game under DOSBox or using a program to slow down your computer.)
There's also the Domovoi conversation. If you didn't talk with the inn domovoi by midnight of Day 5, then you won't be able to get the doll out of the cabinet because it will have mysteriously disappeared. You need the doll to give to Tanya later.
In all fairness, there are about fifty of these in Quest for Glory IV - it has to be one of the buggiest games made by Sierra. There's even one that shows up at the very end of the game — after you have beaten the final boss — if you are playing a Wizard, then the game will often give you the message that you are too busy to cast a spell right now, which prevents you from casting the game-winning spell.
In importing a magic user from Quest for Glory II to the third game, players are given the chance to make them paladins. These players must think of buying the throwing skill (or have the forethought of wishing for it from the djinn at the end of the second game). Lacking the throwing skill makes the game unwinnable if you are a paladin — winning a throwing challenge against Uhura is needed for the Leopardman prisoner event to occur. (An unconverted mage can earn a staff to trigger the event, but a paladin can't.) This only affects paladins who were mages; former fighters and thieves already have throwing.
Likewise, it's possible to import a character as a mage, even if they were formerly a fighter or thief and never bought the magic skill. At least a little magic is required in order to solve some mage-mandatory puzzles. Of course, in any case, the game tries to persuade you away from changing your class on import if you didn't earn it (as with becoming a paladin).
Leisure Suit Larry
Leisure Suit Larry 5: An Unwinnable by Design occurs if you forget to write down the numbers of the various limousine companies you must call throughout the game at the single time they're shown in each location. This leads to an Unwinnable by Mistake: At one location, you can call a different "green card" limo company instead of the regular one, but when you want to return, they won't pick up; only the regular limo can take you back to the airport. If you only wrote down the green card number assuming they could return you, you'll never be able to get back. Worse, because viewing the real limo company number is a flagged event, failing to view it makes the scenario unwinnable even if you use a guide to get the correct number (with Sierra's stock "Oops!" error).
In Leisure Suit Larry 3, there is a bit where you need to lift weights in order to buff up. Now, here is the thing: For unfathomable reasons, the amount of lifts are based on the processing power of your computer. In 1989, you needed to do a leisurely 25 push-ups. On a modern computer, without a mod, it can easily go into the thousands. "Use the cursor keys until you work up a sweat" indeed.
In Leisure Suit Larry 2, a programmer fixed one error, but accidentally caused another one by defining "bag" as a verb. That means that, in the last puzzle, you can't say "Put bag in bottle", which causes Larry to throw the bottle into the volcano without lighting it on fire, making the game unwinnable. You have to specify the bag. "The bag", "brown bag" and "airsick bag" are all acceptable, just "bag" is not.
While later Sierra games tried (and failed!) to avoid unwinnable situations, many contained game-stopping bugs that caused them. For example, at least one version of Daryl Gates' Police Quest: Open Season had a game breaking bug or programming oversight in which, if you failed to show the bone to SID on Day 3 before you gave it to the coroner, then the final scene of the day with the reporters at the morgue would fail to happen, which would make the game Unwinnable. Other Game Breaking Bugs could cause the game to crash on the map screen at the beginning of a day (it would happen every time with the saved game).
Police Quest III had the "endless highway" glitch: if you stopped a speeder or other criminal on the freeway near the "end of your jurisdiction", instead of turning around afterwards, you would get stuck in an infinite stream of stoplights. Hope you have an extra saved game.
Police Quest III had another problem on the highway. On the first day, you are sent to handle a crazy man in the park. After doing so, you can return to the police station and advance the game to the next day. However, you will eventually hit a brick wall where you can no longer progress. This is because, after arresting the crazy man, you were supposed to patrol the highway and give out several tickets, kicking off a series of events that allowed you to steal your partner's locker key.
The Adventures Of Willy Beamish
When trying to get your frog back from Chef Childish, the puzzle is on a timer based on your system speed. On faster computers, it is impossible to complete this puzzle without running Dosbox or Moslo.
On one occasion you need to obtain two items by winning them in the Yahtzee dice game. The trick is that authors incorporated the 'anti-cheating' solution that allows your opponent to accept the game only limited number of times (and calling you a cheater later), thus rendering the save-restore method useless. There is no other way to obtain the items, so if the player character loses all his money, he must start the game over from the beginning. The real kicker? The game does not recognize the straight, so betting a straight against a pair means losing a round.