Wine making is a lengthy process with many variables, each of which has a noticeable impact on the final result. Most wine makes it into the bottle with little issue, but occasionally a batch will run into some problems. These are referred to in the trade as wine faults.
If a wine smells musty or moldy it is considered "corked," and should not be consumed. This happens when a traditional cork, which is made from the bark of an oak tree, is infected with mold. This used to be a bigger problem and is the main reason why many ready to drink wines on the market today have switched to plastic corks or the controversial screw top.
As we previously mentioned, a few bubbles are normal, but if you notice excessive fizzing in your wine, there could be secondary fermentation taking place. Fermentation is a part of the wine making process that converts the sugars to alcohol using a yeast culture. Sometimes the wine will hang onto residual sugar, which makes it unstable. Combined with active yeast that didn't make it through the filtration process, further unwanted fermentation can happen. This is a rare occurrence that is most likely to happen in medium sweet wines.
If wine is exposed to too much oxygen, it will become brown and dull in appearance. Leave a glass of wine out overnight and you can see for yourself what an extended period of oxygenation does to wine. If it comes out of the bottle that way, it's not suitable to drink. Overall cloudiness also indicates a fault, or may just be a poorly made wine.