There have been a few programmers who have started modding existing, largely used, and recently abandoned plugins in efforts to add in backdoors to the plugin. Such an example is the NoCheat+, where a backdoor op command is in place that allows for a user to op themselves.
DO NOT DOWNLOAD ANY PLUGINS EXCEPT THRU DEV.BUKKIT.ORG OR THE DEV'S OWN SITES
This is a security issue that can be easily stopped. If you have downloaded a plugin from the forums.bukkit.org site, it is HIGHLY recommended you download the version from their dev.bukkit.org site to insure you do not have a modded version.