It is possible to tell mdadm to create an md device on a raw disk even though it will give you an error, it writes a superblock and this corrupts the partition table which can result in your system not booting.
To fix it just zero the super-block on the offending device that you made the mistake in.
Eg: /dev/sda
mdadm --zero-superblock /dev/sda
It is also a way of starting fresh if you wanted to create a new array.
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