What are the most common FSM bugs?

Top of the list: unhandled (state, event) combinations, a real input arrives in a state you didn't write a case for, and the machine either ignores it silently or falls through to wrong behavior; the fix is a default case or filling every table cell, even if the action is "ignore" or "go to error." Second: no error-recovery/resync path, real streams are noisy, so a parser that doesn't reset to a known state on a bad checksum or unexpected byte can wedge forever mid-frame; you need an explicit path back to the initial/idle state. Third: uninitialized starting state, if the state variable starts as garbage the machine behaves randomly, so it must be initialized to a defined initial state. Fourth: state explosion, bolting orthogonal conditions onto a flat machine multiplies states until it's unmaintainable, which signals you should refactor into hierarchical or cooperating machines. And for Mealy-style actions, double-firing or skipping transition actions if the action isn't run exactly once per transition. The unifying theme is totality: every state must define behavior for every possible event, including the unexpected ones.