You can breach the screening of even a very good screened room in a number of ways.
You now set up currents in the shell that were not created by an incident wave. And the screened room itself becomes your radiator into the outside world. It upset the people who'd just spent an awful lot of money on two linked chambers.... one for the test subject, one for the test equipment.
So you have to be very careful about cable routing and diligent about keeping all the connections grouped close together where they pass through the skin (and connect their grounds/screens to the skin.
Ferrite beads on cables help a lot in realising the ultimate attenuation a chamber is capable of. For the measuring receivers etc, you soon see the difference between single screened cables (RG213 etc) versus double screened (RG223). You learn that N-types aren't bad IF the braid connection is well made (crimps seem better than compression types) and BNCs simply don't have solid enough ground connection, relying on springs a bit too much.
You learn a lot de-bugging a new screened room.
The biggest scare was coming in one morning and seeing the painter from the building maintenance department just about to start on it.... There were 'do not paint' stickers near the door. Maintenance boss said the colour of the chamber was unacceptable, it HAD to be painted. I said I couldn't argue with that, but give it a couple of months and the new chamber would be scrapped and replaced with a brand new one, because the existing one was about to be ruined... and the new one would be back to the chamber manufacturer's colour. He came round to seeing the point when someone mentioned cross-charging it to his department's budget.
So maintenance erected a partition wall all around the screened room complex, and painted that wall their approved colour!
Of course, it all lasted about 2 years before management decided to move departments around, and the screened rooms had to be moved downstairs
Having seen the price of a GTEM cell (sounds of Serge Gainsbourg and Jane Birkin in the distance
) I've wondered about the idea I came across of getting a lorry trailer type shipping container and welding up all the seams and doors, adding an RF proof door.
Maybe just because I'd enjoy the welding...
David