> [ext4 data= mount option] writeback: Data ordering is not preserved – data may be written into the main filesystem after its metadata has been committed to the journal. This is rumoured to be the highest-throughput option. It guarantees internal filesystem integrity, however it can allow old data to appear in files after a crash and journal recovery.

Not just old data from the same file, either. In the case of file appending, "old" can actually mean "whatever junk happened to be in that allocated disk block earlier".