Prof. Eugene Volokh at UCLA's law school has posted a blog entry at the Volokh Conspiracy about his discovery of an error in Westlaw's report of a case and his unsuccesful effort to get Westlaw (part of Thomson Reuters) to correct it. While I am sure that this error will eventually be corrected, despite the statements of the customer service representative to whom Prof. Volokh spoke, this does illustrate a larger point about legal research.

Although Lexis, Westlaw, and other computerized legal research systems are quite useful, there may be times when you need to go back to the original print sources to verify something you have run across. Even if you don't suspect error, you may need to go back to the original because you want to ensure that a crucial point is correct. Online layout of legal sources is sometimes confusing and may not replicate the formatting of the print source, so you may have confused one part of the source with another (e.g. confusing part of the dissent with the majority's opinion in a case).

And even for sources in print, you may need to verify something with another source. For instance, the U.S. Code is generally reliable, but if you are dealing with one of its titles that is not positive law, you may want to go back to the Statutes at Large to verify a statutory provision if it is crucial to your argument.

And don't forget, even though both Lexis and Westlaw have research attorneys, you can always ask a reference librarian to help you with your research.