Deliberate mistake: The computer Felicity Huffman pushes off the desk is a dummy model and is not connected to anything. When she first pushes the computer off, the monitor only has a power cable visible. These computers were connected via VGA cable that was screwed on; it is quite difficult to disconnect it from the computer. And also when the monitor hits the ground, you hear light sounds of glass breaking and you can see as the monitor rolls that the glass is missing, yet none is to be seen - it's a hollow chassis. Obviously done as the stuff was shoved off in Kelsey's direction so as to avoid injury.
Deliberate mistake: About halfway through the show, Roz and Daphne are in the kitchen talking. Daphne pours water into the teapot - but there is no steam coming from the kettle, nor any sound of a boiling/just boiled kettle. No self-respecting Brit would make tea with water that wasn't boiling.