Our cat Midnight knew that the knob opened the door, but she lacked opposable thumbs. If she could get up to a doorknob (as she could in the kitchen, or in the bathroom, where there were convenient surfaces to climb on), we would sometimes find her banging on the knob with her paw in an effort to open it.
She certainly knew how to open an unlatched door, even if the door had been partway shut – she reached under the door, paw up, then pulled it towards herself until she could reach the edge sideways. None of our other cats ever worked that out – they often accidently shut doors that were partway open, in fact.
My cat Graham was a stray who wandered into my home one evening, sat down and decided she liked it.
When I took her to be spayed a week or so later (when it was clear that she wasn’t leaving), I dropped her off before work and went to pick her up on the way home.
Me: Hi, I’m here to pick up Graham.
Vet Tech: Ah. That one. Hmmpf.
Me: Is she alright? Was there a problem?
VT: Yes, and yes.
Me: ??
VT: Well, after you left I took her in back and put her in a cage. I turned around to get her record to put it on the door, and when I turned back, the door was open and she was on the floor.
Me: Haha. She’s crafty alright.
VT: So I picked her up and put her back in the cage and then watched as she opened the latch and jumped out again.
Me: Wow. She’s smart.
VT: So I picked her up and put her in one of our double latch cages, where one latch goes up and one goes down. The phone rang, and I answered it, and when I turned back the double latch cage was empty! She was on the floor again and I swear she was grinning at me! I put her back in it and watched her work both latches at the same time and get out again! I had to zip tie her in, and even then I kept checking her all day to make sure she hadn’t gotten out!
Like phouka said, thank Og cats are too lazy to be interested in world domination.
We had a cat who had figured out that doorknobs have something to do with opening doors. Unfortunately, she was a bit too short to reach the knob. We’d watch her try when she kinda wanted out but wasn’t desperate. She’d put her paws on the door then hike up, take a step or two closer, hike up again, step again until she was pretty vertical, then s-t-r-e-t-c-h until I swear I could hear the vertebrae in her back separate . . . and still wind up an inch short.
It may be just as well; she would have been dreadfully disappointed to finally reach the magic knob and have nothing happen.
When my mom was young, she had a cat who could turn doorknobs. He’d get a running start, jump and grab the knob with both paws, and use his momentum to swing on the knob. The knob would turn, the door would open, and he’d come flying in.
One day, he made the mistake of trying this trick on the bathroom door when my mom was in the tub. Before she even knew what was happening, he had landed in the bath and leapt straight out the window onto the roof.
And he still had the guts to keep opening doors after that.
Dude, if the call were mine to make, you’d have just won the thread. 
Silly Plant. Everyone knows that Martians are GREEN. 
This thread has just made my day
It is significant that the OP’s name is Psychobunny.
I had to get a new cat carrier with a double latch because Santi figured out how to unzip his old duffle-style carrier from the inside. He hasn’t shown much ingenuity in other areas. I think he just really hates the carrier. From desperation springs innovation.
Our new cat, Serendipity is qutie clever. My wife is gone on business this week, so the cat has been hanging out in the man cave. As it is the man cave where I watch my HDTV, I have heavy black curtains in ehre. The decore is all black and chrome. Anyway, I have a chair for when I play drums in Rock Band, and I sit it in the corner near one of the windows to keep it out of the way when not in use.
I walked into the room after going to shower to find she had managed to paw the curtains open and hook the extra fabric around the leg of the chair so that it would remain so. She was laying on the seat of the chair lookign contentedly out the window. I stared for a moment and she looked at me like, “What? It’s just a curtain.”
I think I am in no danger. Cookie Monster is dumb as a house brick. She has asked to be let in the same door for three years now, but always stands at the hinge side, then looks shocked that she gets hemmed in by the door and has to walk around it. Despite this, she can identify the sound of the can opener being lifted out of the utility drawer - and knows that when I take a teabag out of the storage container, there will be milk involved at some point.
Be wary, jjimm, for your Cookie Monster is slowly lulling you into a false sense of security, so that when she implements her genius plan, you will be unprepared.
Not when they’re covered with Mars dust.