I heard about some possible errors in the process that cause off-flavours (for example skunk due to sunlight). Excluding infection problems, and talking about most obvious errors one can do and are rather easy to identify and correct, what are the most common off-favours a homebrewer faces, what are the causes and what the soutions?
Banana and Bubble Gum
Produced intentionally in certain styles (like Hefeweizen), the flavor is reminiscent of banana candy or "Juicy Fruit" bubblegum. The flavor comes from esters produced when (1) fermenting yeast at a higher temp range than optimal, or (b) under-pitching the yeast, or otherwise stressing them out (not enough oxygen, etc).
Puckering / Tea bag
When the beer has a drying, puckering quality, this is typically due to increased tannins in the beer. There are two main ways tannins can enter the beer:
Fly sparging with water above 5.8pH and temperature above 176°F/70°C, which extracts tannins from the grain husks
Boiling a large quantity of hops for a long time. (e.g. 200g/8oz in a 20l/5gal. batch.)
Here's some great info from How to Brew by John Palmer
Medicinal / Band-Aid
With a flavor reminiscent of cold medicine, or an almost "rubber"-like twang, this one was very confusing to me when I first found it in my Dark Imperial Blueberry Witbier (as wretched a brew as it sounds).
There are two commonly accepted sources of this flavor: (a) bleach and (b) wild yeasts. The beach you get from not properly rinsing bleach-based cleaners/sanitizers. The wild yeast you can pick up from improper sanitation practices.