It is possible to save an incomplete or erroneous (Q)Form, which means that it is possible to save (Q)Forms that are not fully finished or that still contain invalid content. The error message of the (Q)Form Editor will be stored with the revision and shown in the description pane, which is part of the revision object.
The revision of an erroneous (Q)Form is tagged as failed, cannot be made accepted and therefore also cannot be promoted to status published. In other words, an erroneous (Q)Form cannot be used by running models.