I ran into a problem with Grails on Google AppEngine. When I was saving my entities it tried to save the wrong type of entity. I ran out of ideas on this one and brought it up on the Grails User mailinglist.
I started to work on some reproduction code and then I went to Denmark for some vacation. When I get back however, Keith Thomas(http://www.nabble.com/user/UserProfile.jtp?user=652771)'); // ]]> had run into the same issue and figured out that it had something to do with the constraints on the domain classes.
Sure enough removing the constraints fixed the issue with saving. Ill try to see if the problem is specific to some kinds of constraints, since I still have my reproduction project to test it in.