A node in an FBAS can fail in many ways. The following Venn diagram copied from P.7 of the SDF white paper) shows different types of node failures.

Byzantine failures

• Examples
  • A malicious user modifies the software and attempts to double spend money.
  • A computer crashes due to a power outage.
  • The network in the area stops working.

Blocked

TODO

Divergent

Suppose $V = \{ v_1, v_2, v_3, v_4 \}$ and $Q(v_1) = Q(v_2) = \{ v_1, v_2 \}$ and $Q(v_3) = Q(v_4) = \{ v_3, v_4 \}$. Even if all of them follow the protocol without any issues, $\{ v_1, v_2 \}$ and $\{ v_3, v_4 \}$ will end up with different ledgers.