My colleague was shutting down RAC instances because some important HW maintenance on servers which Oracle instances are running on. He issued as usually "shutdown immediate" command and database was closed and dismounted correctly and also the instances had stopped. Quite normal... But during start of instance at the mount phase there was following error occured: 


ORA-201 controlfile version incompatible with ORACLE version
Ok, never faced this error in the past so I checked the oerr and the output was: 

00201, 00000, "controlfile version %s incompatible with ORACLE version %s"
// *Cause:  The controlfile was created by incompatible software.
// *Action: Either restart with a compatible software release or use
//          CREATE CONTROLFILE to create a new controlfile that is
//          compatible with this release.
Hmmm... that's very interesting, the error message said that "controlfile was created by incompatible software" but we didn't change anything... Also as you can see in the message that I got there is missing the arguments (compare the message with oerr output). OK, what about messages in alert.log? I can say that only alert.log gave to me the real picture about problem... 

***
Corrupt block relative dba: 0x00000001 (file 0, block 1)
Fractured block found during control file header read
Data in bad block -
 type: 0 format: 0 rdba: 0x00000000
 last change scn: 0x0000.00000000 seq: 0x0 flg: 0x00
 consistency value in tail: 0x00000000
 check value in block header: 0x0, block checksum disabled
 spare1: 0x0, spare2: 0x0, spare3: 0x0
***
ORA-201 signalled during: ALTER DATABASE   MOUNT...
As we are using multiplexing controlfiles (of course) the solution was quick. Just to be sure I used the md5sum command and checked all three controlfiles and only the first had different checksum. Since I work with Oracle I saw lot of situations when controlfile was corrupted but usualy the error messages were ORA-00227, ORA-00202 but for first time with ORA-201 which is IMHO misleading in this case... Question is why one of control files become corrupted when DB was closed and unmounted clearly and without any problem and why we got ORA-201 (instead of ORA-00227, ORA-00202) which is misleading (IMHO) in this case. Also it is very strange when the error message is missing additional parameters. So trust to your alert log and of course use the multiplexing of control files (I hope that actually you are)...