With a nod to the very excellent Very British Problems, I've amassed my own little list with a twist. Mine has less to do with our ingrained awkwardness and everything to do with being a fish out of water, i.e. a Brit in the USA:
Ordering tea and forgetting to say "hot". Especially in the South, you're as likely to get iced tea as you are hot. And be prepared for them not to understand you when you say "hot".
Ordering hot tea and forgetting to ask for milk. I've had it served with a slice of lemon, sometimes honey, occasionally a small jug of cream, but never, ever plain old milk.
Running out of tea bags. Yes, they sell "British" tea bags here but please. Talk about gnat's pee. You soon learn to ask every visitor to bring a suitcase full of teabags.
Having to hide your taste for instant coffee. I have actually met Americans who have never had instant coffee. Admittedly, the instant here isn't great but Americans are usually appalled that I even have it in the house.
Trying to order water. The different pronunciation of the "a" and the "t", followed by the non-rhotic "r", makes this request a nightmare and is usually where Brits break down and fake an American accent. Or just break down.
Not being able to use Dettol everywhere for any event. I love Dettol! What can I say! Nothing comes close here but I have yet to meet an American who is not completely revolted by the scent.
Not being able to use Germolene on my kids' cuts, because Ditto. Yes Germolene stinks too, but that's how you know it's good. I mean who can trust an ointment with no smell? (I'm looking at you Neosporin.)
Boasting about your Blue Peter badge and no one batting an eyelid. I know someone who has one and I'm well impressed. Sadly since she's also a Brit-in-America, she doesn't get the respect she so clearly deserves.
Complete strangers imitating you. Yes, it happens. I'll be standing at the counter, handing my money over, minding my own business and the sales assistant will break into what can only be described as a cross between the Queen and a constipated member of the Monty Python team. And I don't even sound like that.
British actors attempting American accents. When you've been in the USA long enough, you can tell a fake American accent from the real thing. It's too bad the actual actors can't.
Vice versa. Actually, when you compare American actors to the woeful Dick Van Dyke performance in Mary Poppins, they come out smelling like roses.
Your own "American" accent. After two decades in the USA I still can't do a decent American accent and I don't know many Brits who can.
Stalking people in shops because you think they might be fellow Brits. A non-scientific study over the years confirms that many of us do this. None of us are ever quite sure what the plan is should the stalkee turn out to be British either.
Trying to make conversation with British tourists who didn't come all this way to meet Brits. OK, that's a bit harsh because sometimes they want to know how long you've been here etc. but on the whole, they're clearly of the mind that Brits are ten a penny back home.
Mixing with Brits you'd run a mile from back in Blighty. Another one we're all guilty of, especially if we've joined a Brit group that meets up regularly. There's always at least one person who gets right up your nose.
Voice automated customer service. This can reduce grown Brits to tears. It's particularly hard to make yourself understood to the robot on the other end when, like me, you have an "R" in the middle of your name.
Not being able to answer online "security" questions because the choices aren't relevant. My bank currently has three security questions I can choose from - my high school mascot, my junior prom date, or the baseball team I followed as a child. Hmmm.