Ran across a problem the other day and thought I might share the solution since it was something that I was unaware of.

The basic situation was that I was trying to create a button that would save the current response document, open a new response document to the same parent, and prepopulate the new document with data from the response that was just saved.  The method I employed was to create a $$Return field that set the QueryString as follows:

"[/" + @WebDbName + "/ResponseForm?OpenForm&ParentUnid=" + ParentUNID + "&from=" + @Text(@DocumentUniqueID) + "]" 

Then I created a WebQueryOpen form to pull the fields from the document that was just saved.  Everything was working as planned, but the response documents were not getting created.  After banging my head against the keyboard for longer than I care to admit, I finally figured out the problem.  Here is the new formula:

"[/" + @WebDbName + "/ResponseForm?OpenForm&from=" + @Text(@DocumentUniqueID) + "&ParentUnid=" + ParentUNID + "]"

Notice the difference?  It seems that Domino required the ParentUnid argument to be the final argument in the QueryString.  Something to keep in mind when you are doing something more than an @Compose to create a response document.