I've read (I think on wikipedia, but I can't find the reference) that microwave oven leakage is often due to faulty seals on the door. Given that any gap in the door is going to be far smaller than 10% of the wavelength, how is it possible for the microwaves to leak through? Shouldn't the Faraday cage function perfectly?
Two things are going on. First - the conductivity of the shielding of the microwave oven is "only so good": the skin effect in particular results in the electrical currents running in only a very superficial layer of the conductor, and this means there is some phase lag (and incomplete reflection, i.e. some transmission of power).
Second, gaps do occur. In a well designed microwave the gaps have compliant conductive seals; but if these lose their springiness I can see how currents flowing around the gap will result in re-emission of microwave energy. You don't need a 2D opening: just a gap that is "long and thin" is sufficient to act as an antenna. Where "long" is a few cm (1/4 of a wavelength). See for example this diagram:
from http://www.interferencetechnology.com/in-situ-emc-testing-using-surface-current-sense-wires-1/ which describes some of the techniques behind using such an "antenna".
The gap in the door is smaller than a wavelength - but the length of the door is conveniently a few wavelengths. So the sharp metal edge of the door re-radiates the energy, like an antenna, through the gap.
Microwave ovens have some special structure on the door edge to create a reflection in the wave and block and leakage - sorry don't know the details, was never into antenna design
The edge of the doors and area around the door on the microwave oven side when closed during operation act like microwave notch filter. The notch filter is tuned for 2450 Mhz. Any deformation of the door by a dent of a insignificant amount could cause the microwave to leak rf energy as it changes the notch frequency. If you are in front of it at the time...God help you. A notch filter blocks a small range of frequencies but passes frequencies above or below.
One experiment you can try that is pretty cool. Put a DECT 6.0 cordless phone inside the oven (without turning on the microwave obviously). Use the base unit to find the phone ...it will ring inside the closed microwave oven. The DECT 6.0 phones operate in the 1.924 GHz range..