Gaping armholes

Threads pulling loose around a cut buttonhole usually means the stitching was too loose, the fabric wasn't stabilized, or it was cut open before sewing finished. Interface the area and cut open last with sharp scissors.

A buttonhole that won't let the button through without straining the fabric usually means it was sized to the button's diameter instead of its circumference. Measure the button's girth and test on scrap before cutting.

Fabric pulling tight and creasing across the crotch of trousers usually comes from a crotch curve too shallow or a rise too short for the wearer. Lower the curve or lengthen the rise in a future muslin.

A facing that buckles or shows on the outside instead of lying flat inside usually means it lacks understitching or the interfacing is too heavy. Understitch close to the seam and grade the allowances.

A neckline that gapes and has lost its shape usually comes from cutting on the bias without stabilizing, or handling the edge too much before finishing. Stay-stitch the neckline immediately after cutting.