Oxidation leads to a stale, cardboard-like flavor in your beer.
If you encounter this issue, there are a couple of possibilities:
If you have stored your beer for an extended period of time, it may have undergone oxidation. Although the bottle cap offers a nearly airtight seal, air can still seep into the bottle over time. If you intend to age your beer in the bottle for an extended period, using oxygen-absorbing bottle caps will be beneficial.
The other potential cause of oxidation is exposure to high temperatures. If you store your beer at room temperature without air conditioning, it may undergo oxidation during the warm summer months.
To prevent this, you can store it in a cooler area, such as your basement floor, or simply refrigerate it.
Unfortunately, once your beer has undergone oxidation, there is no way to fix it.