When you attempt to save your registered field changes, List Fields might encounter errors. For example you try to convert an existing text field with the content "This is a test" to a number field. The conversion won't succeed since "This is a test" can't be converted to a number.
If the save operation encounter any errors, you will see this dialog box:
Note that List Fields will process all change log entries sequentially, and all previous successful changes has been saved.