I have built many forms in InfoPath 2007 and you always find new interesting issues to deal with =)
When you think you have found a brilliant solution, then you can be sure that there is an obstacle when it comes to web enabled forms. So it works with filter on data sources.
I wanted to drag and drop a secondary data source to my form, but I wanted it to show only values that matched a field value in my form. Let me clear: only values in the data source that belonged to the current user should be displayed in the repeating table. Easy, I thought and clicked the “Value” button and then the “Filter data…”. Got this error message:
Hmm after hours of trixing with setting the field from other fields and so on, I disabled the web enabled layout. Now you may press the “Filter data…” button and set your filter. Web enable the form again, publish it and just press the “Ignore error messages” and continue with the publishing…. It worked!
So that error message is BS. I have saved the form, and all data remains in the form.