One of the most frequent and unforgivable mistakes made in legal writing is that of citation bias; whether done inadvertently or intentionally.
This is not a problem of "making up" citations, but rather a problem of selective citation or improper paraphrasing of sources.
Even the most ethical of writers can make mistakes or even inadvertently become biased with one side of an argument. But, unfairly citing the legal sources that conflict with your argument (including entire sources or portions thereof) can do great damage to your argument and reputation; the following citation errors, whether intentional or inadvertent, should actively be avoided:
Inadvertent Citation Errors and Biases
In preparing for any type of legal writing, it is necessary to research all sides of the legal questions at issue. Whether you are writing a legal memo for your boss, or are directed to write a research paper in defense of one side of question, you need to know the strength of the other side's argument. During the process of legal research though, it is easy to become enamored with one side or the other or just give up after you find a few relevant sources.
Making either of these mistakes, and not performing thorough research, will likely lead to a skewed interpretation of the legal rules surrounding the issue; and will cause your conclusion to be at least inaccurate if not completely incorrect.
Imprecise Quotation of Sources
Quoting sources is an art to be mastered. Anyone can quote a source, but to quote a source efficiently and accurately is sometimes extremely difficult; but especially so when quoting from long-winded sources such as court holdings.
It is far too easy to pick and choose beneficial quotes from a large source without delving deeper to ensure that the speaker didn't mean something completely different than what the selected text implies. It is imperative that you completely understand the reasoning of your sources before you quote them as a legal source.
Biased Paraphrasing of Sources
Due to the length of most legal resources, it would be impracticable to quote all of the legal reasoning within each source; thus, paraphrasing can be a powerful tool.
As with imprecise quotations, though, it is extremely easy to misconstrue the reasoning of a legal source with a few ill-advised words. Before paraphrasing a source, it is necessary to thoroughly understand the reasoning of that source. Without this thorough knowledge, it is impossible to accurately encapsulate pages of reasoning into a few sentences or paragraphs.
Intentional Citation Errors and Biases
Improper Omission, Quotation, or Paraphrasing of Sources
As tempting as it might be to ignore or change sources that conflict with your side of a case, it is always best to accurately include all sources.
If you are in a professional setting, the omission or skewing of relevant sources will not only discredit you as a professional, but the subsequent (and faulty) legal reasoning could also negatively affect a client or employer.
On the other hand, if you are a student, such omissions will be heavily frowned upon by your professors and your grades will reflect that frown; this is because they will know that you either didn't study or were too arrogant to realize that your professor has seen such parlor tricks before.