In case you didn't read the comments, here's what was offered.
Fiona said not knitting the right size--which is terrific and something I should have remembered as a frequently-offered answer.
Collegeknitting said not creating enough ease or checking ease for other than the bust.
Wildflower said knitting without personal modifications, especially with respect to length.
Suncat said not personalizing fit.
Carolyn said being afraid . . . to knit what you like, to customize a pattern.
So, all responses were about fit. But before acknowledging how close these are to the words that flew out of my mouth, I'd like to address what Carolyn said.
Believe it or not (and as Carolyn seems to know), I hear knitters say they are afraid--to knit a sweater, to rip, to change parts of a pattern. And my response is that fear and knitting are two words that should never occur in the same sentence. There are many things in life to fear, but knitting should not be among them.
Okay, so my answer was about fit. My response to "the most common mistake knitters make" was . . . they follow the pattern! Who said that?!?! I write patterns!!!! Why would I say that???
. . . because there are places on a pattern that must be changed.
When we chose a size, it is solely based upon girth (and on the girth of the largest part of us the garment will cover--so good idea to check schematics other than bust). The patterns does not know how tall you are, or how long your arms are, so, every pattern should say SHORTEN OR LENGTHEN HERE, telling us 1) to do something and 2) where to do it.
In addition, if you shorten or lengthen, then no cardigan pattern should read pick up and knit 137 stitches along the front edge!!!!!!! If you shortened, if you lengthened, or if your row gauge does not match the pattern's, this number is not helpful and probably incorrect. Instead, the pattern should tell you the proportion with which to pick up (maybe 3 stitches for every 4 rows) and then the multiple needed for the edging's stitch pattern (maybe 4 + 1 stitch).
I routinely write about and teach pattern modifications. And here's a story I always tell when doing so.
A woman from my old guild knit The Gray Cardigan from MOTHER-DAUGHTER KNITS--a pattern that instructs not only to adjust for length but also for shoulder width. This woman made an exquisite version of this garment, and the first time she wore it someone said "What a gorgeous sweater! It looks hand knit, but then I realized it couldn't be because it fits you too well."
There's more to say on this subject, but that's enough for today!
Thanks for your comments and I welcome more plus any questions you may have.