I've stitched a design that came out just fine, except for the outlines. They don't line up, and there are gaps in some places. How can I fix/prevent this?

This is the oldest problem in embroidery. When a design is sewn, the stitches will tend to pull the fabric along the lines of the stitch angle. This can cause gaps in alignment, and because outlines are typically quite thin, the gap is very obvious. It falls on the person doing the digitizing to prevent this problem. Using a technique called pull compensation, the outline is placed in such a way that it will stitch correctly. With free downloaded designs, this technique is almost never used. Commercial embroiderers are much better at it, and the problem is much less likely to happen in a design that you purchase.

Fabric choice is another area where this problem can occur. When a design is digitized, many adjustments can be made, especially in the areas of pull compensation and underlay. (Underlay is the "base" stitch that gets sewn in filled areas, and then covered by the fill stitches.) Since the digitizer has no way of knowing what fabric you will be stitching the design on, most of them use a "middle of the road" approach and digitize for dress-weight woven fabric. When this type of design is stitched on a stretchy knit, problems are almost inevitable.