mdadm did not notice a failed disk in raid0

The md(4) man page sheds some light on how the word "clean" is used (crucial bit italicized):

Unclean Shutdown

When changes are made to a RAID1, RAID4, RAID5, RAID6, or RAID10 array there is a possibility of inconsistency for short periods of time as each update requires at least two block to be written to different devices, and these writes probably won't happen at exactly the same time. Thus if a system with one of these arrays is shutdown in the middle of a write operation (e.g. due to power failure), the array may not be consistent.

To handle this situation, the md driver marks an array as "dirty" before writing any data to it, and marks it as "clean" when the array is being disabled, e.g. at shutdown. If the md driver finds an array to be dirty at startup, it proceeds to correct any possibly inconsistency. For RAID1, this involves copying the contents of the first drive onto all other drives. For RAID4, RAID5 and RAID6 this involves recalculating the parity for each stripe and making sure that the parity block has the correct data. For RAID10 it involves copying one of the replicas of each block onto all the others. This process, known as "resynchronising" or "resync" is performed in the background. The array can still be used, though possibly with reduced performance.

If a RAID4, RAID5 or RAID6 array is degraded (missing at least one drive, two for RAID6) when it is restarted after an unclean shutdown, it cannot recalculate parity, and so it is possible that data might be undetectably corrupted. The 2.4 md driver does not alert the operator to this condition. The 2.6 md driver will fail to start an array in this condition without manual intervention, though this behaviour can be overridden by a kernel parameter.

It's plausible that a disk in the RAID failed after the RAID was safely and normally disabled by the system (at, e.g., a shutdown). In other words, the disk failure happened with the RAID in a consistent, synchronized state. The RAID would then be flagged "clean", and, when it was next enabled and one of its disks failed, the flag would remain.