In online games, particularly MMORPGs, it is common for the game developers to release "content updates" or patches to a game in order to update it. Sometimes these updates introduce unexpected behavior into the game, or more rarely, such unexpected behavior already exists in the game but is only discovered well after it was introduced. Some unexpected behavior, such as corruption of image files, connectivity problems and display issues shall be referred to as non-destructive bugs, as they do not alter or destroy any of the character or item data that represents the game world. Other unexpected behavior can be referred to as destructive bugs which do cause damage to character or item data.
The non-destructive bugs type tends to fall into one of two categories: technical flaws and oversight errors.
Technical flaws are bugs that actually break a game mechanic. In Dofus, game mechanics would explain how the world works and how characters interact. Therefore movement taking a character through trees, exchange windows item duplication, or the ability to die in battle without losing energy would be seen as technical flaws. These types of bugs are obvious to any experienced player as game mechanics work different than they historically have.
The second type of destructive bug, oversight errors, arise from normal game data following normal game mechanics producing results unexpected by the game developers, and often harmful to game balance. Examples of oversight errors in Dofus include the farming of flax and subsequent sale to NPCs for kamas (Please see this wikipedia article for details on gold farming), the high drop rate of kamas on Bonta/Brakmarian guards, the prespic sets ability to nullify mushd and piglets for easy xp, the unexpected high drop rate of O'Lanterns combined with their NPC sale price of 5000 kamas, or the previously overpowered nature of the Ecaflip's AoN spell.
Technical flaws and oversight errors are not all equal, as some can be fixed by simply changing how the game works (nerfing AoN, lowering the sale price of flax to 1) while others produce difficult or impossible to reverse effects on the game world once they are introduced (the flood of kamas from the O'Lantern incident, exchange-window item duplication).
The more destructive bugs can cause the game developers to perform a Rollback, which resets the game world to a previously saved state, erasing anywhere from a couple of hours to days of game activity as if they did not exist.