When I turn on my computer without it connected to the internet Steam tells me that that there is not connection and prompts me to choose between "offline mode" and "quit".
If I click on "offline mode" I get the Steam error: "This action cannot be done in offline mode"

So I can't get in offline mode if I'm not already in offline mode ?
What nonsense is this ?

you can work around this by Snatching out your UTP cable (internetcable) and then connect to Steam, might need a few tries but its possible

To enable offline mode Steam needs to sense that everything is up-to-date and ready to play. Games that aren't installed don't count against this. If you were connected previously, Steam could have sensed patches that were available and disabled offline mode.

You are currently in online mode with no Internet source, not offline mode.

I think if you exit steam in online mode it wont ask you to go offline

anyway this is what I do

Dont let Steam start up when windows does
make sure you run the game at least once in online mode
always go to offline mode before exiting Steam

Dont even go online unless your playing online / updating
and remember always go to offline mode before exiting Steam

This works for me but I have got that error before