7 Errors
Syntax errors like incorrect or missing punctuation while using curves will result in TEX or LATEX error messages. The TEXbook and LATEX manual explain the meaning and correction of these errors. The previous examples and Section 9 should make the correct syntax for curves commands clear.
curves will write a LaTeX er+ror. message to the screen and log file if you supply an incorrect number of coordinates.
If four sequential points in a drawing command argument have the line through the first and third parallel to the line through the second and fourth:
exactly or closely, curves knows it cannot draw a parabola tangent to two parallel lines, issues to the screen and log file:

LaTeX Warning: \curve straight from ...

and draws a straight line;
or approximately, curves may draw an unexpected curve with no warning.
If four sequential points in a drawing command argument have the line through the first and second parallel to the line through the third and fourth:
curves draws a parabola which may be nowhere near the curve.
If the four points were on a straight line, removing one or more points is a remedy. If they are not on a straight line, adding points may help. Specifying many points will give you a satisfactory curve with perhaps an annoying number of \curve straight warnings. After a \curvewarnfalse, curves still uses the straight lines but does not tell you.
Curvature changes sign on curves like . Specifying inflection points as
curve coordinates will reduce error and specifying sufficient coordinates will then give satisfactory results. For discontinuous tangents splitting a curve into pieces is unavoidable. Splitting a curve into pieces with curvature the same sign can give satisfactory results with fewer coordinates. \tagcurve can prevent tangent discontinuities. If an inflexion point's exact location is unknown, try the midpoint of the straight line through the ends of its segment.
Curves appear rougher than horizontal and vertical lines. Picture digitization causes this not inaccuracy in TEX,
curves.sty or curvesls.sty. Setting \diskpitchstretch to a value less than one with \renewcommand may smooth an unusually rough curve.
Symbols and symbol spaces misaligned are usually due to rounding error. Adjusting
\diskpitchstretch below one can reduce rounding error and increase alignment accuracy. This should be limited to the misaligned curve with { }.
The replacement
\bezier does not give exactly the same results as the original in bezier.sty or in LaTeX2e. The difference is extremely small but if it is important to you comment out the five lines of code for \bezier and \@bezier near the start of curves.sty or curvesls.sty. You now have a \bezier which is slower and needs more memory but has only its original capabilities and gives only its original results.
Please email me examples of any errors not covered above. You may have found a bug in the code or documentation.