Circuits that are connected to outside world should be reasonably protected. Who knows when somebody just plugs in a different power adaptor (of course with twice the voltage and polarity in reverse). So how to account for this?
You can use a diode in series and zener diode (or better – a transient voltage suppressor) in parallel in combination with a fuse. But you can do it better. First, the diode in series can be replaced by P-channel MOSFET (Q1). Drain connected to input and source to output. Gate is connected to ground. The transistor appears to be reversed and it is. But on purpose. Suppose the voltage is reversed. Then, gate is on V+ so the transistor is closed. Its intrinsic diode is reverse biased so no current flows.
During normal operation, the current flows through its intrinsic diode but the gate, being grounded, has also lower voltage than the source so the transistor conducts in active mode. It is fully open and its resistance is the Rds(on) value. Which can be very low. In this way, the diode drop is eliminated and energy loss on transistor is negligible.
If you need to protect against voltages that are larger than the gate-source breakdown voltage (Vgs), then a voltage-limiting zener diode (D1) and a current-limiting resistor (R1) have to be added to the gate.
It is possible to automatically disconnect a circuit when the input voltage exceeds a predefined level. This is different from parallel transient voltage suppressor and fuse. Blow fuses have the obvious problem of being a one-time devices. PTC resettable fuses tend to be slow and they can conduct significant amount of current even in tripped state (to stay tripped, the fuse needs to maintain itself hot). You can use electronic fuses for overcurrent conditons though.
With just two P-MOSFETs and a few components, it is possible to automatically limit the input voltage. The principle is this: A zener diode (D2) is chosen with value slightly lower than the maximum allowed voltage on the output. Then, when the voltage is higher than the zener voltage, D2 starts conducting and voltage drops across resistor R2. This lowers the voltage on the gate of Q2, which will open, raising the voltage drop across R4. The gate of Q3 sees higher voltage and Q3 thus closes, disconnecting the protected circuit from excess voltage. All without tripping any fuses or causing excessive current flow. Neat! Because you can get P-MOSFET with very low Rds(on), the losses can be minimal.
Resistor R3 and two zeners, D3 and D4, are there for gate protection just like D1 and R1.