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Bad game design snack
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Most likely, especially with large scale games, there are game-play bugs scattered throughout a game. It's acceptable if the bug doesn't negatively effect game-play too much. However, if there is a bug with the core mechanic--or verb--then that game shouldn't be shipped until that bug gets fixed. For example, if you are making a game that is FPS, then you better make sure that there are zero bugs with shooting. Otherwise, this will ruin the entire game since that is what the player will do the most.
XCOM 2 is a turn-based strategy game where the player must strategically place his troops in the field to take out the enemy unit. Placing your troops at strategic vantage points is key to this game. This game has a critical bug with enemy field of vision. For example, say you have a troop hidden behind a corner of a building, out of enemy sight. On the enemy's turn, sometimes they are still able to see your troop and attack them through the wall. This just ruins the game for so many players.
Terraria is a 2D platform adventure game. One of their selling features is that you are able to dig under the ground (see the screenshot below) and explore. However, I had the situation where I digged a bit underground and it would not light up around my character (which it should). Then I would lose sight of my character and moving the character would not work at all. I had to restart the game. Given that players go underground for a considerate portion of the game, this bug definitely affects core gameplay.
Example 03: Assasins Creed
They had a huge problem with bugs, mainly on the Black Flag one. It is annoying for the player to get so many bugs and have restart the game all the time. For example, a player jumped from one boat to another and somehow fell IN the boat and not ON the boat, so the player got trapped inside the 3D modeling of the boat and it had to restart the game. This sort of situation happened a lot and got players irritated and annoyed about the game situation. The game designer needs to make sure the game is working fine, because it is not a "playtest", but a final release.