Stains. Well this may not have been your fault! Stains can have different origins: smoke, water problems or not priming properly before painting. You need to determine the cause of the problem before you fix it.
You can clean the spot and surrounding area with TSP and then let it properly dry. Afterwards, put some stain blocker over the stain and let dry completely. Only then, apply another coat of paint.
Lighter/Darker Coverage. Did you ever wonder why the color on your wall is not the same as the one you originally bought? It will most definitely be because the wrong number of coats have been applied. Some clumps or streaks may appear darker because of extra paint in the edges of the rollers, and will result in a patchy appearance. This can be fixed by sanding those patches and recoating as many times as necessary to even out the color.
Drips. This is the result of poor technique when painting and we don’t blame you! You live and you learn. Overloading the brush will make the paint drip and leave those marks on the wall. It’s an easy fix: you just have to get a scraper to take off the paint drips; sand the affected area to remove any sharp edges and repaint. Usually just one coat will do it.
Wrinkles in the paint. This one is a tough one. There’s a reason why they tell you to let it dry! Wrinkled pain can be caused by putting coats on top of each other without being completely dry. Other factors that can affect this are high temperatures while drying.
You will need to strip the paint all together using a hot air gun or a stripping product. Then sand it down and repaint.
Filler showing. If you can see the patches, it means that you didn’t prime the filler patches properly (or at all!) before painting. Especially with water-based paints you will have to either recoat the patch or sand off the paint, prime the patch and repaint.
Bleeding knots. When painting over wood, you need to be careful of knots and prime them before you apply the paint coat. Otherwise, the resin in the wood will keep secreting and it will result in stains. If that has already happened, scrape the pain until you hit the knot, apply sealer and then recoat.