A couple of RMAN errors that took me a few minutes to figure out

Problem:  When restoring a database to a new host with RMAN, I ran into the following errors.

Error 1:
On restoring a control file to a database started in nomount, and listener up:

ORA-19870 error reading backup piece
ORA-19504 failed to create file
ORA-27040 file create error, unable to create file.

This error is pretty cut and dried.  Either the file path does not exist, or the oracle user doesn't own/have permissions to create the file.

Error 2:
on restoring a database to a new host with the control file restored, database in nomount, and listener up:

Failure of recover command at
ORA-01013 user requested cancel of current operation

This error could be much more clear.  It can be caused by an open connection to the database.  Since you're already restoring the database, the easiest way to remedy is to shutdown the database, then startup nomount, and make sure you exit your session before re-running from RMAN.