This event occurs only if the instance is mounted in shared mode (Oracle Real Application Cluster).
During the read of the buffer, the contents changed. This means that either:

The version number, dba, or the incarnation and sequence number stored in the block no longer match

The checksum on the block does not match the checksum in the block

The block will be reread (this may fail up to 3 times), then corruption is assumed and the corrupt block is dumped in the trace file.