It’s possible to have data corrupted on a HAMMER2 volume during a specific combination of a bulkfree operation and a lot of writing to disk. Matthew Dillon has a potential fix already. As he announced, it’s scheduled to go into 5.4 this weekend. It’s a rare bug, but if you want to check for it, look for CHECK FAIL entries in /var/log/messages.
And because every cloud has a silver lining: some not-yet-quantified performance improvements.