If you opened a file, and in the meanwhile the connection to the server was lost, and you don’t have CSC enabled, any further attempt to read/write will fail.
DFS Failover is only performed when the client opens a file or folder. It does not apply for files or folders that are already open. In this case, the application will receive a failure on the operation.
If you have CSC enabled, and the file is backed by the CSC cache, the attempts to read/write will operate offline. The application won’t receive an error, and the changes will be sent to server once the connection is reestablished. If a further create is done and the system realized that the target for the DFS namespace is now available, CSC will attempt to backpatch the opens to the server if the open has not been modified. (And the new server could potentially be different from the original server if DFS failed over.)
Technet original documentation: