Steps to reproduce:
- call a new Dynamic Web Project wizard
- choose Web module 2.4
- add the GWT facet
- set the Java facet to the 1.5 version if that is required
- click Finish
You will get an error dialog as in the attached screenshot.
Solution: Creating the GWT servlet for Web Module 2.4 or changing a declaration of the GWT facet to require Web module >= 2.5