I get this error when trying to publish a browser-enabled form. The full error is "The filter expression is on external data and could not be validated. It may not be supported in Web browser forms."
This is happening because I have cascading drop-down lists that are populated from a Sharepoint List. The second drop-down filters its list items based on user selection in the first list. I guess Infopath can't properly validate the filter because the data exists on these external lists but there must be some way to do this.
I read
Clayton Cobb's answer and see that I might be able to add something to the Form Load rules, but my list is much more complex than the example given.