If you retrieve a persistent domain instance and set one or more invalid properties in a controller and attempt to save the instance in a transactional service method, validation isn't performed and the instance is flushed. If one of the validation problems involves a database constraint (e.g. nullable) then the call fails due to a database constraint violation. I've attached a sample project to illustrate. Create a Thing instance and then edit it. Set the name to NULL, which forces the value to null since empty form fields are blank, not null. Rather than validating the instance and logging the messages, you'll get an error page with the error I spent some time debugging this a while back and it looks like the call to the services getMetaClass() ends the transaction and triggers a flush, so the call to thingService.saveThing() never actually executes - you can see that from the console log messages. I saw this in a 1.0.4 app and it's a problem in 1.1, but I haven't tested in 1.0.5.