We see the same mistakes from many students new to Organic Chemistry, so we have provided a collection of the most common. Look through the following links to a make sure you do not repeat the epic mistakes that we see over and over again that cost students points.
Texas Carbons - Never, ever draw 5 bonds to carbon
Labels and bonds - connect the right atoms in the structure
Fixing Cyclohexanes (How to avoid having axial and equatorial go horribly wrong)