RMAN recovery question

A cold backup was taken of a 10.2.0.3 database using rman. The database 
was shut down cleanly before the backup was started. When I tried to 
restore the backup a few days later, it would not let me open the 
database. It said that file 1 (system01.dbf) file needed further 
recovery. Why would a file need *any* recovery if it was a cold backup?

These are the rman commands used to do the restore.

startup force nomount;
restore controlfile from '$mediahandle';
alter database mount;
restore database from tag $tag;
alter database open resetlogs;

The $variables are shell variables that specify the appropriate 
information needed to restore the correct control file and backup sets. 
It's part of a script that's worked dozens of times before. Everything 
worked fine up to the "alter database open resetlogs;" where it started 
complaining about needing recovery on one of the files. There were no 
errors in either the backup or the restore.

TIA