The file will be killed, no error message. But there would be an error if you try to kill it again (when it no longer exists). However, the user who opened it still has a read-only copy in memory on their machine -- it doesn't disappear before their eyes and I think they experience nothing at all. If they close without saving it is gone. But if they try to save the file, they will have the option of saving it with a new name (note: a user would not be able to save the file with the old name b/c Excel will not allow this ... *but* a user *could* save a copy with a temporary name, and then re-save that copy with the original name, thus restoring the file).
(My co-workers are used to me experimenting on them
)