Not using face primer/eyeshadow primer/setting powder/setting spray
This can lead to makeup not lasting as long as it should, creasing, and applying patchy. This will also cause your foundation to settle into your fine lines and pores and draw unwanted attention to your pores. Also not setting it will keep your makeup from lasting all day. It can even cause it to slide off your face if you have oily skin and don't use a primer to control your oil levels.
Not prepping to change textures on face
You can't use different textured products on your face without setting the other first. For example if you use a cream/liquid foundation then go in with a powder contour without setting your face with translucent setting powder first, that contour will never blend out. The color will just stick in the foundation where applied and not budge. Same thing goes for eyeshadow primer and eyeshadows. If you dont set the lid with an eyeshadow close in shade to your skin before starting, you will have an awful time trying to blend. After you've switched to powder products, do not go back over with a liquid or cream and try to blend it. Likes blend with likes.
Not using the right product for the look you're trying to achieve
Sometimes you set yourself up for failure by using the wrong product for a look. Are you trying to achieve a sharp winged liner with a kohl or gel pencil eyeliner? It just won't work. Look at what you are using and looks that can be made with that, instead of trying to achieve a borderline impossible with that tool creation. Also you can just buy yourself the right product which in this case is a liquid or gel liner.
Not blending your makeup and using the wrong tools
99% of the time you go out with unblended makeup, you look like a hot mess. Your makeup should blend and have no harsh lines. To do more complicated eyeshadow looks it often requires a base color, brown bone high light color, transition shade, crease shade, and lid color. The transition shade helps flawlessly diffuse the dark shade from the crease. If you're using brushes just do a simple window wipers motion back and forth. Sometimes you can't blend because you're using the wrong tools. This can be those little sponge applicators for eyeshadows or your hands for tasks such as contouring.
Not washing your brushes regularly
Not sure why your face is breaking out? When is the last time you washed your makeup brushes. The brushes can become a breeding ground for bacteria with the mix of your facial oils and makeup buildup over time. It just gets nasty. Also you're stuck to one color shade with that dirty brush because the leftover makeup product color on the brush will deposit right along with the new shade you're trying to use, leading to a muddy look.
Not trimming your fake eyelashes or blending them after applying them
False eyelashes are not one size fits all. There are so many different eye sizes and shapes out there. Most of the time the band will be too long for your eye. Trim from the outside edge in. If you trim from the inner edge, you are cutting off the shorter lashes transition into the longer ones and you'll have a very unnatural look not blended. Speaking of blending your lashes, after you apply them and let the glue set, take a little bit of mascara on your real lashes and push it up into the bottom of the falsies. Do not pull mascara through the falsies. Also you'll want to cover the band better by retracing your eyeliner.
Not using small natural strokes to fill in eyebrows or not blending them
We've all seen our fair share of bad eyebrows. Almost all of these mistakes can be fixed by applying the brow makeup (whether it's a micro pencil, gel, pomade, or shadow) in small short hair like strokes. This mimicks the natural look and helps it look most natural. Also never underestimate the magic of a spoolie brush. Drag that through a few times if you were too heavy handed and it will help drastically blend it out.
Doing too much work trying to remove mascara oopsies
Did you sneeze or accidentally boop some mascara onto your finished look? There is a super easy trick to remove it that doesn't involve messing up the underlying makeup at all. Take your spoolie brush and rub it on the DRIED mascara and it'll come right off.