Something I see a lot, with people who are new to sewing, is a lack of understanding of the grain of woven fabric, and why it's important to cut out pattern pieces the way the pattern tells you to in relation to that grain.
The grain runs parallel to the selvedge edges of the fabric yardage, and is physically made up of the warp yarns that run the length of woven fabric, that the weft yarns are...well, that those yarns are woven around to make the fabric.
Because the warp yarns have to withstand the shuttles, carrying the weft yarns, passing back and forth across them repeatedly, the warp yarns are generally stronger than the weft yarns.
This means that, if you cut something perpendicular to the grain that is intended to be cut parallel to the grain, then the direction of the soft yarns and the direction of the sturdy yarns will be swapped, and the piece won't hang like it's supposed to.
Furthermore, the warp yarns need to be more stable, while the weft yarns, since they're traveling back and forth while being woven, are more flexible, which often results in a subtle stretch across the grain--from selvedge to selvedge--that can make a noticeable difference in how the finished item fits, especially if it's in any way fitted. There is almost never stretch along the warp/with the grain.
I so often see people new to sewing doing things like folding the fabric so the cut edges are together, which results in cutting the pattern pieces from the fabric across the grain instead of with the grain (not to mention doing potentially unwanted things to the nap and print direction.) Folding the fabric so the selvedges are together is usually what's directed in pattern cutting layouts, with the pattern pieces laid out parallel to the folded edge and selvedges, along the grain.
It's also common to see new-to-sewing people laying out the pattern pieces in all directions (usually jokingly called tetrising), to maximize the efficiency of fabric usage. This can easily lead to a finished item that doesn't quite fit right, even if you've used that exact pattern with that same kind of fabric before, and, without understanding the role of grain, it will seem completely random as to why it didn't work this time.
I know that a lot of the things sewing patterns tell you to do seem fussy and unnecessary, but they really do have their reasons!
