I had a very nasty experience with Oracle ASM recently. A long story short, ASM crashed on both our RAC nodes when removing a corrupted archive file on file and incarnation number (removing it on it’s name didn’t work). Starting the instance failed. When removing the failing diskgroup ASM was happy again. Luckily the failing diskgroup only housed archivelog files, except for one database it also held it’s index datafile.  So that one wouldn’t come up. If you’re in such a situation, you can use the Oracle tools amdu in combination with kfed to extract the missing datafile or archivelogs (in our situation) from a dismounted diskgroup. If I knew that from the start it saved me alot of frustrating hours.