It turns out that view-change, at the core of Fast Practical Byzantine Fault Tolerance, may be trickier than it seems.
In this ArXiv note (https://arxiv.org/abs/1712.01367), we observe a safety violation in
Zyzzyva and a liveness violation in FaB.
To demonstrate these issues, we require relatively simple scenarios,
involving only four replicas, and one or two view changes.
In all of them, the problem is manifested already in the first log slot.