safe state machines

Quartus is not minimizing the states rather than the connecting logic, with the result, that some illegal state codings may be non-escapable. You can check this with the Quartus FSM template design. In default one-state-hot coding, it uses four state variables. The FFs (with the input signal as clock enable) are simply connected to a chain with two inverters.

You can immediately identify two illegal state codings "1000" and "0111" that are non-escapable, cause they have an identical successor.

P.S.: I apologize for the picture quality. Unfortunately good quality gif files are resampled to bad quality jpg by the forum software.