Question: What exactly happened, could be observed by you? I sometimes observe the following behavior on my phone, when I flash my builds to test: The boot animation normally is pretty slow, but lasts short - sometimes after a new flash, the boot animation has normal/fast speed but seems to run "forever". I don't really wait in that case, but simply press [power]+[vol-up]+[vol-down] to shut it off. Then I normally enable the device again and it boots fine.
In that specific state however, after the above force-shut-off, if you launch fastboot to switch from a to b or vice-versa (hoping to boot the last version), it will tell you after boot that /data would be corrupt. Ignore that and switch again to the previous partition and simply boot. Don't ask me, why - but that is, what my device sometimes does... - anyways, don't immediately give up, when the system tells you /data would be corrupt, spending some time playing around is worth the effort ...