So, we ran into a nasty bug last night. We are running 10g (various releases) RAC on 3 or 4 node clusters. In this particular configuration we had a 4 node cluster, with an instance for this database on each node. 2 instances were active, two were configured but not running.
DBA went to make redo log adjustments (adding a new group) and database crashed. There is a bug in 10g (and apparently 11g) with respect to this kind of configuration. If you are running an active/passive kind of RAC configuration, you will want to read up on the bug. Be very careful making any online redo log changes if you are running in such an environment.
Metalink bug number is 6786022 and it's public. We understand patch is in QA to correct. There is also an event you can set to avoid the problem. See the bug on Metalink for more information.
So, check out the bug and make sure your system is not vulnerable.