There are essentially 3 things that constitute defects in a bottle of wine, commercial or wine made from kits. If you notice it in a restaurant, or purchased commercial wine, send it back.
 
Corked: 
A wine is said to be corked when it has come in contact with a contaminated cork during the ageing process. The results of the contamination are almost always unmistakable. The wine will smell like a wet basement after a flood or dirty socks left in the hamper a little too long, nasty and not all enticing to the taster. On the palate, it will be astringent, lacking in fruit, with a raspy finish. Sometimes you may even notice a paint-thinner quality.
 
Oxidized: 
Oxygen is wine’s invisible enemy, and when a wine gets exposed to air, it becomes “oxidized”. The result is flat, lifeless wine that loses its pretty, vibrant fruit scents and tastes insipid – it will likely remind you of vinegar. The trained eye will also often notice a certain dullness in colours. In whites, it can be light to dark yellow or even brownish.
  
Re-fermented: 
Occasionally, some residual, dormant yeasts will wake up, and a wine will undergo a second fermentation after it has been released and shipped. This manifests itself as effervescence, or fizziness, on the tongue.
 
Excerpt from Winexpert’s files.