Hah! See block list at Dark's pool. Even if official explorer list claims, that my block hash at 84230 is correct, Dark's pool has unlock his invalid block!
I saw same at your pool and our test pools. Blockchain says that block is "ok", but after 10-15 seconds its got orphaned by network. This started after big hashrate increase, from 90-100kh/s to 600+kh/s.