This bug affects people who using the practice of keeping customizations to modules such as DSpace-API in the /modules/additions folder according to the docs at:
These customizations will be packaged as additions.jar and end up in the WEB-INF/lib directory together with the standard dspace-api and other JARS.
Right now, there is no custom classloader or other mechanism that guarantees that the servlet container will use the classes in additions.jar instead of the other ones. Tomcat v5-v7 happen to load the jars alphabetically, and because additions.jar comes first, these classes get loaded.
This is not part of the servlet spec, nor documented in Tomcat:
http://stackoverflow.com/questions/5474765/order-of-loading-jar-files-from-lib-directory (answer with 18 pluses).
This bug will not manifest for people using Tomcat v5-v7 but WILL result in problems for people on Tomcat 8 according to this report:
(Won't fix - Tomcat says applications, like DSpace should deal with it, and not Tomcat itself)