We had the following schema in production.
And then we executed the following alter.
And one row (that we can tell) got corrupted at the same time and could no longer be read from the Python driver.
And then in cqlsh when trying to read the row we got this.
We tried to read the data and it would refuse to read the name column (the UTF8 error) and the last_pin_timestamp column had an absurdly large value.
We ended up rewriting the whole row as we had the data in another place and it fixed the problem. However there is clearly a race condition in the schema change sub-system.