If:

a) You are using Apache Struts and Tiles

b) You have multiple Struts configuration scripts like so:

...
<init-param>
   <param-name>config</param-name>
   <param-value>/WEB-INF/conf/struts-default.xml</param-value>
</init-param>
<init-param>
   <param-name>config/module1</param-name>
   <param-value>/WEB-INF/conf/struts-module1.xml</param-value>
</init-param>
<init-param>
   <param-name>config/module2</param-name>
   <param-value>/WEB-INF/conf/struts-module2.xml</param-value>
</init-param>
...

3) You use

<html:form action="/someAction">
...
</html:form>

4) And you get a "javax.servlet.ServletException: Cannot retrieve mapping for action '/someAction' at [insert a massive stack trace] " error

Then:

5a) Check your web.xml file to see if the configuration resources in (2) are declared properly

5b) Check your struts-config-xx.xml file to see if you've mapped "/someAction" correctly

5c) Check your tiles-def-xx.xml to see if you've defined the tiles and the put values correctly

5d) Replace:

<html:form>

with the basic:

<form action="/module1/someAction.do">

to confirm that the action is working

6) As a last resort, remove the moduled implementation of the configuration scripts like so:

...
<init-param>
   <param-name>config</param-name>
   <param-value>/WEB-INF/conf/struts-default.xml</param-value>,
   <param-value>/WEB-INF/conf/struts-module1.xml</param-value>,
   <param-value>/WEB-INF/conf/struts-module2.xml</param-value>
</init-param>
...

7) Check to see if this solves the error in (4). If it does, remove all context references (module1, module2 etc) from your actions.

This took me the better part of two working days to figure out, so maybe this will help some frustrated soul out there. If you know a better solution, bully for you. I shall stick to my comma-delimited lists, thank you very much.