While editing a calculated custom field on a custom form in Setup you get the following error
"'Custom field name' field is used in a multi-form configuration, if you would like to change this formula you will need to remove this field and replace it with a new one containing the desired calculation"
Remove the field from the custom form and replace it with a new one containing the desired calculation
The error occurs because of the following: currently you have an object (or objects) in your system that has both the form you are editing and some others containing the same calculated field attached. As you can't have the same calculated field with different calculations on the same object, the system doesn't allow you to make a change which will result in calculations being different.
For example, you have a task with Custom forms A and B attached to it. They both contain the same calculated field, Field 1. Now if you go to Setup and try to edit the calculation of Field 1, let's say on Custom form A, you will encounter the above mentioned error.
You can identify the objects causing that problem by creating a report on that object and adding Category Name field in the view. For tasks the field is Task -> Category Name (and not the Category -> Name which shows only the primary custom form)