With the many deprecations related to the ActionErrors and ActionError classes, there are a variety of questions relating to:
To summarize, the difference between the classes ActionErrors/ActionError/ActionMessages/ActionMessage has absolutely nothing to do with the difference in behavior in Action.saveErrors(...) and Action.saveMessages(...)
The difference between the classes is zero -- all behavior in ActionErrors was pushed up into ActionMessages and all behavior in ActionError was pushed up into ActionMessage. This was done in the attempt to clearly signal that these classes can be used to pass any kind of messages from the controller to the view -- errors being only one kind of message.
The difference between saveErrors(...) and saveMessages(...) is simply the attribute name under which the ActionMessages object is stored, providing two convenient default locations for storing controller messages for use by the view. If you look more closely at the html:errors and html:messages tags, you can actually use them to get an ActionMessages object from any arbitrary attribute name in any scope.
While we're clarifying, the difference between html:errors and html:messages is purely in syntax and model -- both tags *default* to look for an ActionMessages object under Globals.ERROR_KEY despite the difference in names. I wasn't part of the history, but I'm assuming that around the same time that people were realizing that there's more than one kind of message to pass, they also realized that sometimes you want more flexibility in displaying them. html:messages provides more flexibility at the cost of more typing. Niall Pemberton has posted more details on the differences between html:messages and html:errors on his website.
I hope this helps to clarify things. I would strongly encourage people to have a look inside the Struts source code, as it's really quite clear when you look under the hood. You can see what happens in validation by examining the "processValidate" method in RequestProcessor: http://svn.apache.org/viewvc/struts/struts1/trunk/core/src/main/java/org/apache/struts/action/RequestProcessor.java?view=markup
You can see what happens with saveErrors and saveMessages by examining those methods in Action http://svn.apache.org/viewvc/struts/struts1/trunk/core/src/main/java/org/apache/struts/action/Action.java?view=markup
You can see what the tags do by looking at their respective source files: http://svn.apache.org/viewvc/struts/struts1/trunk/taglib/src/main/java/org/apache/struts/taglib/html/MessagesTag.java?view=markup http://svn.apache.org/viewvc/struts/struts1/trunk/taglib/src/main/java/org/apache/struts/taglib/html/ErrorsTag.java?view=markup