5. Wet Cardboard
Beer that has been oxidized can instill aromas and flavors reminiscent of cardboard, wet paper or just a general stale characteristic. While oxidation can sometimes take the form of sweet and sherry-like qualities, adding depth to some styles like barley wine or old ale, it is rarely a desired trait in beer. Unfortunately, once oxidation occurs it is unable to be fixed, but steps can be taken to prevent if from happening in your next homebrew.
The key to preventing oxidized beer is avoiding the introduction of oxygen after fermentation. In fact, in most cases you should avoid aerating wort after the oxygenation at the time of pitching the yeast. Oxygen is initially important to yeast because it consumes it to aid in growth and preparation for the fermentation ahead. Once the yeast gets to work, their consumption of oxygen decreases, which is why its important to avoid aeration. To do this, ensure the wort is not splashed or agitated after fermentation, especially when racking. Keep airlocks full. Ensure fermenters, kegs and bottles are all sealed airtight. And if possible, purge kegs, bottles and fermenters with CO2 before racking fermented wort into them.