Unwinnable By Mistake/Sierra

King's Quest

 * 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.

Space Quest

 * 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: DOS Box. Just use this with an appropriately low number:
 * Space Quest II has a similar bug when Roger swims through a pond.
 * Space Quest IV 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

 * 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 DOS Box 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 ). 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: 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, then you're still up Unwinnable creek without a paddle.

There is a related bug that can be triggered whose ramifications are not fully realised until later; any savegames made after the bug is triggered are Unwinnable. After arriving at any destination airport, the player must find a means to summon a limousine to travel to a target location. A phone number for a local limousine company may be found in every airport: this is the number that the player must memorize or write down because calling it again is required to return to the airport and advance the plot. But in one airport, a number for a "green card" company may also be found. If you call the green card company number first, then Larry will request limousine transportation, which will arrive and take Larry to his next destination. But the green card company number cannot be used to transport Larry back to the airport; it simply will not work, and it will be treated like any other invalid number. If you have left the airport without even seeing the number for the limousine company, then your game has been rendered Unwinnable. Moreover, because viewing the limousine company advertisement at the various airports is a scored event, and thus a game flag, failing to activate this flag will cause the game to error out (with the rare but uniform "Oops!" error used in many Sierra adventure games), making the scenario likely Unwinnable even if the player has obtained the proper phone number from a guide.

Police Quest

 * 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.