Custom list with content type item check-in throws "Value does not fall.." error

When checking in an item in a custom list, I was being presented with the following error:

Value does not fall within the expected range. at Microsoft.SharePoint.SPFieldCollection.GetFieldByInternalName(String strName, Boolean bThrowException) at Microsoft.SharePoint.SPFieldCollection.GetFieldByInternalName(String strName) at Microsoft.SharePoint.SPListItem.get_MissingRequiredFields() at Microsoft.SharePoint.ApplicationPages.Checkin.OnLoad(EventArgs e) at System.Web.UI.Control.LoadRecursive() at System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint)

After much head-scratching and nightmarish debugging, I found that the answer was very simple: Do not have the "Required" attribute set to true in your content type definition, but rather specify this in your custom list schema.

Hope this helps!