I borrowed my mother’s new car to do her grocery shopping for her. I parked in the enormous lot, went in, shopped, came out and - yep, forgot I had driven a different car (a black one, not the gold one). I walked around that parking lot pushing a cart for 20 minutes, searching in vain for my vehicle. I kept circling back to where I had parked, though. The third time, the light dawned - “D’oh!”
I have probably too many to count, but here’s one from college:
My university used magnetic locks on all of the doors. Typically you used your university ID card to gain access, and hit a large red button to release the lock to get out. It became instinctive for me to hit the red button whenever a door failed to open.
So one time I was working on a project which required access to the engineering department’s VAX mini-mainframe. The room was small, cramped, and climate-controlled. I went to exit the room, and the door stuck, so I instinctively hit the large red button next to the exit.
It was the emergency shutdown for all of the computers. D’oh!
We have one of those. I use it for plates but our bowls don’t really fit under it. Plus, for something like oatmeal where it just boils over and out all over the rotating platter, it’s not going to help much anyway. But yeah those are good for splattery stuff.
at least you didn’t get halon flushed, …
… DID YOU???
Thankfully no.
But I have seen other computer rooms with halon systems. They generally seem to take more effort to activate (like putting them under a plexiglass cover outside the room with prominent warning signs).
In my case the issue was that it was a huge PITA to restart everything that was interrupted without being properly shut down. A small thing with modern computers, but a much bigger deal with those ancient devices.
He has a long-standing habit of keeping a notebook for fuel fill-ups that includes the date, odometer reading, gallons, and price. He also has a spreadsheet on his computer that makes use of this info for various calculations (mpg, cost per mile, etc.)
I’m not sure I see much of a point, but he does. It’s a “pick your battles” kind of thing.
I used to do that years ago. But all my recent cars calculate mpg themselves. My Prius also allows you to enter the fuel cost every time you fill up, but I don’t bother so I’m not sure what it does with that information.
before cars had all those fancy computers in them (avg. gpm, etc…) I had a nice app on the cellphone where you only had to enter the current odo-reading and the amount of liters you bought gas (in order to fill it up) … and it did all those fancy-schmancy calcs for you …
again, obsolete today …
I was returning the cart they have downstairs for packages that are too heavy to carry. As I swung it toward the door, it hit the emergency button, which called the elevator company and set off the front desk alarm. Fun!
I suspect that, for people like @Seanette’s Designated Hitter (that’s what DH stands for, right? 
), that’s all the more reason to keep records: to check the car’s accuracy.
The H is for Husband. D can be anything from Darling on down, depending on how boneheaded he’s been recently. 
That’s a relief. At first I misread Thudlow_Boink’s suggestion as “Designated Hitler”.
Today’s D’oh (dough?) moment - I was making a batch of molasses cookies but the dough just wasn’t coming together, it seemed too dry. I checked the recipe about three different times to see if I was missing something, when I finally noticed in the middle of the list of dry ingredients “4 Tbsp molasses”. 
Yeah, that might be helpful if you’re making molasses cookies.
That’s almost as good as the directions for making a backyard nuclear reactor included in Tom Weller’s “Science Made Stupid”, which mentioned adding the control rods after completing the fuel assembly.
We went to the Big Island of Hawai’i for my Mom’s 70th. She’s always been a crummy driver and that hasn’t gotten better with age. She had rented the car, but I (somewhat snottily) told her I’d drive. We wanted to get groceries before going to the condo, and I deftly pulled into a parking spot at the market. And hit a pylon. D’oh!
Major “d’oh” moment last night. I took a TV watching break to use the bathroom attached to our bedroom. I don’t bother turning on the bedroom light because I know the layout of the room so well. So when I exit the bathroom and bedroom at night (especially in the dead of winter) I exit the bedroom door in pitch darkness, as I’ve done maybe thousands of times.
Except last night I went wide right for some reason, and WHAM I walked face-first into the door frame. Ouch! Luckily, other than a red welt on my cheek today, I didn’t suffer too much lasting damage. Other than to my pride and self-respect.
Went to order a sandwich at Jimmy John’s. The missus wanted a ham and cheese with no mayo. So instead of ordering it with no mayo I, for some unknown reason, ordered it with ‘no ham’. D’oh!