Unwinnable by Mistake/Tabletop Games/Board Games

Warhammer 40,000

 * It is possible to win a game of Warhammer 40,000 by killing all of the enemy units that can claim objectives, making the game unwinnable for that side. Other units can still contest objectives, so a draw is still possible.
 * This was built into one of the armies, the Necrons, who used to just disappear if more than 75% of their army is utterly destroyed. Note that "army" in this case means the total number of units with the "necron" rule; there were a considerable number of units without this rule. This superseded all other official mission rules.
 * Close Combat with certain units. Due to the "you cannot wound creatures with a toughness that's 4 higher than your strength" rule and the "roll a D6 and add the result to your strength for Armour penetration" rule, it is very possible for you to run into combat with a dreadnought or a Wraithlord, only to realise there's absolutely nothing you can do other than wait till every single one of your models in that unit is curbstomped to hell. 5th edition got a little better where even the most basic units carry Krak grenades, which are strong enough to hurt Dreadnoughts (but still not Wraithlords). The Soul Grinder is even more dangerous, as it has an Armour rating of 13, meaning that unless you brought dedicated anti-armor close combat weapons against it, not even your grenades will help you (and in some cases, the weapon wont even help).
 * There is also an army-wide variant of this. Any wise opponent would bring at least some form of anti-armor against you, especially if they know you to use tanks. However, if you destroy all their heavy weapons, it's very possible to win by default (only in certain missions, however, like Annhilation). This is especially obvious with the new Imperial Guard Codex, where most of the MTBs have an armor rating of 11 on the back, meaning most people would be unable to even break those tanks in close combat, making the game very literally unwinnable.
 * Deepstriking units are not placed on the table and randomly come in during the game. However, if all your available troops are killed before the Deepstriking ones arrive, you automatically lose since you have no more units. This can be caused by a considerably bad roll, but is much more pronounced for Daemon Armies, where the entire army must deepstrike, which stands a very good chance that either all of them would get shot to hell in the first turn before the rest of your army comes (you can only shoot or run on the turn you deepstrike and only one type of unit in the army can actually shoot, meaning the entire army is slowed by at least a turn), or you scatter into other units and/or off the table, destroying what little troops you start with and fork over an automatic victory to your opponent.
 * If your opponent has some form of mobile unit and all you have is units with Rage, you can never win. Rage forces the unit afflicted with it to run towards the nearest enemy unit, which means if your nearest target is outside of your maximum move + run range (usually 12") then you won't get near enough to cause damage. Your opponent can then use his turn to move another mobile unit into Rage's area and move the other out of it, causing the unit to run straight towards the other one. In this way, your opponent can play "tennis" with your units while pumping gunshots into them, claiming objectives round the board and generally screwing around with no risk of any injury. And to make matters worse, the units acting as the "rackets" are usually vehicles, meaning that unless you somehow get in range to assault them you will be unable to damage them because Rage forces you to move, forbidding the use of heavy weapons.
 * A hilarious one can happen if you decide to hold your entire army in reserve and there are no deployment restrictions (only possible in certain gametypes): Your opponent can deploy his entire army near-touching your board edge (infiltrators can deploy even there, without any units to keep them at bay) and your units can no longer legally come on. This is because they must legally move the entire unit onto the battlefield without getting within 1 inch of an enemy unit or breaking through their formations, which you can no longer do. This actually happened — search for "Kroot Conga Line".

Other Board Games

 * In some board games, it is possible for a player to be in a situation where there are no legal moves to be made, such as stalemate in Chess. In contemporary chess, a stalemated game is a draw; historically there was no standard rule, and stalemate was sometimes considered a loss for the stalemated player -- or sometimes a win (like in Chinese Chess). Chess also has a rule that the game is drawn if no possible sequence of moves from the current position can lead to a win. The most obvious example is when both players have only their king left, but there are other possibilities, such as the 16 pawns forming a complete blockade, that are covered by the rule even though they will never arise in realistic play.
 * The 50-move rule (the game is a draw if no pawns have been moved and no captures have been made after each player has taken 50 consecutive moves at any stage of the game) was added because it was thought that the game was Unwinnable when it had devolved into such a state. Then someone found a way to mate a player this way...
 * Dragon Realms has the potential to create this for one or more players and make things very annoying for the others. If a player is very low on cash, a natural disaster like a flood can destroy enough of their railroad that they can't afford to repair it and are cutoff from any city where they could make more money. As a last resort they discard their contract cards and draw a new set of contract cards hoping to get one that will get them the money to proceed. However, this increases the chance that another disaster card will be drawn which only makes things worse. They have legal actions in the game but those actions get them nowhere. The other players now have to deal with the possibility of a natural disaster card every other round instead of every five to six rounds. Also having one player sit around for another hour or two locked in an unwinnable situation is not a pleasant experience for everyone. Since lending other players money is not allowed, the others players will find a way to pay the stuck player rent money for using their railroad just to get them back into the game.
 * In the board game Hero Quest, it is entirely possible to lock the game into an unwinnable state by making either the Elf or the Wizard use the spell "Pass Through Rock" then passing through one of the many boulders that are used specifically to stop you from going to rooms to have no way in and nothing of interest thus trapping you on one side of the board with no way out.
 * An Obvious Rule Patch changed it so that if you "Pass Through Rock" and end your turn in one of the empty rooms or hallways, you are considered to have ended your turn inside solid rock, and died messily.
 * In some Hero Quest games, the rules state that the players can only search once in a given room. This can lead to some of the quests becoming unwinnable.
 * Betrayal at House on the Hill boasts fifty Haunt scenarios that are randomly chosen each time you play. However, due to the random nature of the game, it's sometimes possible to end up in a situation where one side literally has no chance of winning. For instance, the Traitor becomes a near-invincible monster with one weakness... only by sheer chance, they happened to find that item and were carrying it when the Haunt started. Leaving the heroes with no way to retrieve it. To make matters worse, some of the scenarios as originally published had conflicting or unclear rules, which could also render a scenario Unwinnable. However, since the nature of this game is not very competitive, in most such situations reasonable players will elect to veto the haunt in favor of something more fun.
 * It says something this game ended up getting a 20-page errata book to correct all the errors, and some are still there.