If you have the 3.8 software, you may have encountered a small bug in certain cases where you get a proposal but then it gives you an error. You reboot the case and on your models the restoration you designed is there, floating in the air. I've seen this happen during crown proposals using the buccal bite. What you can do to get rid of the crown is go back to the image catalog and remove all the images from your prep model and put the images in the yellow SAVE folder. Then bring the images back and re-create the model. This essentially resets the model and allows you to get rid of the floating crown. This will be fixed in an upcoming patch, but for the time being, if you get this weird proposal, just reset your model, and all will be well.