Running out of fabric when making a gored skirt in stripes that had to be matched (many years ago in Home Ec.) The teacher had me make a short triangular insert of white with several horizontal rows of narrow cotton lace and sew it into the skirt at the bottom, tiny bow at the top of the point or apex (think that is the right term). I've used that idea many times through the years to make a neckline look better.
Underarm gussets when I didn't do a test fitting and the blouse or dress didn't give me enough room to move my arms.
These days, most of my clothing construction involves re-vamping ready made.