Broken or rough outlines

Four causes, in order of likelihood

1. The bitmap is too small. Below roughly 800 px on the long edge, individual pixels become visible as steps in the curve.
2. JPEG compression. Faint blocks around edges create specks and ragged borders. A PNG of the same artwork traces cleanly.
3. The threshold is off. Too high and the outline swallows the anti-aliased fringe; too low and edges break into fragments.
4. Background is not clean. A grey gradient or a shadow behind the object becomes part of the path.

What to do

1. Re-export the source at a larger size, or ask for the vector original — it is often available.
2. Convert to PNG and delete the background before tracing.
3. Trace again with the standard preset, then move the threshold in small steps and watch the preview.
4. If only a few specks remain, delete those paths by hand. Fighting the threshold to remove three dots usually damages the good edges.

When to redraw instead

Logos with fine lettering below about 3 mm, or photographs, are better redrawn or traced in a vector editor. Auto-tracing a photo produces a large number of paths that no machine enjoys cutting — see Accuracy vs node count.