Here's a great post by David Walker at the Law Librarian Blog about reliability problems with online citators including Westlaw's Keycite, Lexis' Shepards as well as open source upstarts like Precydent (previously discussed here).
I'll let you read David's very detailed post discussing his impromptu experiment with online citators to determine the validity of United States v. Napier, 233 F.3d 394 (6th Cir. 2000) in light of the U.S. Supreme Court's recent holding in District of Columbia v. Heller, 128 S.Ct. 2783 (2008). Suffice it to say - impress upon your students that these services are far from foolproof or even accurate.
Hat tip to Joe Hodnicki.
I am the scholarship dude.
(jbl)