Protecting a 3.3V circuit from 5V power supply
Here is the type over voltage protection circuit I would use if it was essential to protect against a greater than 3.3V input. This circuit uses a low cost voltage comparator to compare a 2.5V reference voltage against a divided copy of the input voltage and then will turn off a P-channel MOSFET when the input voltage goes above 3.3V. 

You can change the reference voltage part to some other similar part at another voltage if needed and the scale the voltage divider resistors accordingly. One example of a low cost reference is a TLV431. A low voltage Zener diode could also be used but the precision of the circuit would be lost and the voltage divider would have to be tweaked to allow for greater margin above the 3.3V level.