There are sewing mistakes that can be explained in two lines and still keep happening. It is not lack of attention: some things are understood with the eyes and the hands, not with a paragraph. These are the classics (and if you are just starting, it will help to go over the most common mistakes before you start sewingas well).
1. The zip that ripples
What happens: the zip ends up wavy and the garment puckers.
Why: the fabric stretched while sewing, or the zip tape was sewn while pulling.
How to avoid it: baste first, do not push or pull the fabric and let the machine feed it on its own.
2. The fabric that stretches
What happens: neckline or armhole edges end up too long and gape.
Why: stretch or bias-cut fabrics give way when handled.
How to avoid it: stabilise with interfacing or tape, and do not let the piece hang while you sew. Respecting the grain direction helps too.
3. The sleeve that will not fit
What happens: the sleeve cap has too much or too little and gathers form where they should not.
Why: the armhole and the cap measure differently and that difference has to be distributed carefully.
How to avoid it: baste a small gather inside the seam allowance and spread it with pins before sewing.
4. Seams that do not lie flat
What happens: the garment looks bulky and never “hangs” properly.
Why: the pressing steps between seams are missing.
How to avoid it: press each seam before crossing it with another.
5. Skipped stitches
What happens: the machine leaves gaps in the seam.
Why: a worn needle or the wrong type of needle for the fabric.
How to avoid it: change the needle at the start of every project and use the right one for each fabric.
Why they are so hard to see in writing
Because they are mistakes of gesture: where you put your fingers, how fast you press the pedal, how much you stretch. A photo or a text only explains it halfway. When someone sees it happen on your machine and corrects it on the spot, you understand it for good. And if the problem is not the sewing but that the garment does not fit you well, see why some patterns do not fit well.
Better seen live
In the workshops you fix mistakes on the spot, with your hands on the fabric.