The column name you have entered is already in use or reserved. Choose another name

I have faced very interesting issue last night.One of the users had changed / renamed the default content type “Title” to “Item Title” and when he tried to pull it back it gave an error – “The column name you have entered is already in use or reserved. Choose another name”

After modifying default name to custom name he realized that “Title” name had been changed to all over sites because on site all lists / libraries were inherited from parent content type. As a farm admin I even tried with no luck and got the same error.

I googled the issue and very quickly found resolution on - http://blog.weareon.net/fix-for-the-column-name-that-you-entered-is-already-in-use-or-reserved-choose-another-name/

There is a small JavaScript we have to run in browser which clears all the values in the forbidden column collection array and gives you a confirmation

And after running following JavaScript I got rid of that issue.

javascript:g_FieldName={};alert('Successfully cleared forbidden columns');