One of the most frequently seen mistakes I see in both my own work (yes, still!) and my students is muddy color. Particularly when you are working wet on wet, it is very, very easy to work unwanted color into your blending, which ends up looking sort of like the image on the right. eeww! To fix? Scrape it off and start over. I know. Sucks.
Ahhh, that old clashy color thing. Please, people--don't forget to think about your color choices! Refer to your color wheel. Never place two highly chromatic hues side by side; Use high contrast only sparingly and only where you want to focus the viewer's gaze. This applies to all types of art whether abstract or realistic.
Another common mistake: applying thin over thick. Oil paint needs a thinner, less motile base on which to add more pigment. Try to add paint onto a ground of freshly laid in full bodied color and what to you get? A mess. To correct: blot the heavy first layer with a paper towel. Better yet, always paint thick over thin.