So the other day, I decided to make myself a ham and cheese sandwich for lunch. Easy enough, I had some nice deli-sliced ham, and old cheddar cheese, and mayo. And I had a tomato, perfect for slicing, and suitable for a sandwich. And a kaiser roll, my favourite. I prepared the sandwich, and enjoyed my lunch. Sort of.
Two bites in, I realized I had forgotten to include the cheese. D’oh!
What are your “D’oh!” moments?
The first few times I baked banana bread, it came out as a hard brick. That was how I learned that baking soda is in the recipe for a reason.
I asked my friend where my scunchi was.
It was in my hair.
I keep my most frequently used spices conveniently located in the cupboard: Onion powder, cinnamon, and nutmeg. The first for use on sandwiches, the next two to alternate between on my morning bowl of oat groats and butter and maple syrup.
One sad morning when I was not alert enough, I seasoned my oat groats with the onion powder instead of the nutmeg or cinnamon. It provided a flavor that I had not acquired a taste for. Nor do I plan to acquire said taste for. 
I often fail at basic door-opening technique 101-- my hand will sometimes slip while turning the doorknob or lever, but before my brain can register this fact, I still try to push through as if I had successfully turned the knob, and face-plant ungracefully against the unyielding door.
Here’s a good one I just thought of-- I was driving down to Florida from Michigan with my kids, 7 and 10 at the time, to see my sister. We had gotten to Florida but still had a long ways to my sister’s house. We stopped at a rest stop for lunch and bathroom breaks. The rest stops on the highway in this part of Florida were large buildings in the median area between the north and south-bound traffic, full of restaurants and shops. I took careful note of where I parked, since it was a large lot.
After lunch and other business taken care of, we were going back out to my vehicle, but it was not there! I look all over the lot despite the fact I knew exactly where I had parked, and it was just nowhere to be seen. I push down feelings of panic and start thinking of what to do next-- call police, call sister to get a ride, etc…
…when I notice that the outside of the entrance used to have a vendor selling bags of oranges, but no longer did. That’s when I realized that the north and south-bound highway sides of the building, and the north and south lots, were mirror images of each other, and I had exited the wrong side 
I’ve done that, as a more experienced cook than you apparently were at the time. Just a brain glitch, apparently.
I’ve also done things like freak out because I couldn’t find my keys - while driving a mid-1990s car that required a physical key in the ignition.
Then there was the time I had fueled my car, finished the mileage notebook DH insists on, and put the car in gear, then couldn’t figure out why it wouldn’t start. Turns out that starting the engine helps a great deal. 
(In my defense, it was after midnight of a day that had begun around 6am and featured DH breaking his leg at work. Very hectic day, loads of stress.)
In my University days, one year I had a summer job at a local factory.
At break time, I went to the works cafeteria for a cup of tea.
On each table there was a bowl of white crystalline powder.
‘A curious way to serve the sugar’, I thought as I spooned it into my cup.
A few moments later: this tea tastes really peculiar??
The powder was, of course, salt…
I was a patient at a hospital around 25 years ago. The ward had a small tea/coffee kitchen for the patients. Each week, one of the patients volunteered to look after the kitchen and keep it clean, filling the dishwasher etc. When it was my turn and I had turned on the dishwasher, I left the room for a cup of coffee, when a few minutes later someone entered the kitchen and yelled out “OMG, what a mess!” I ran to the kitchen and saw that the whole room was filled with foam, about a meter high. Turned out that I, who didn’t own a dishwasher and had never operated one, had filled it with liquid detergent for hand cleaning instead of the proper powder or tab for the dishwasher. Oops, the kitchen cleaning that day took a lot longer than usual…
On a trip to Paris, I missed my transfer on the Metro because I misread a sign that said “CDG ORLY” (the names of two airports at opposite ends of the line) as “CDG ONLY.” I still remember the moment I realized “wait a minute, the word ‘only’ is English…but they speak French here…”
The battery in my garage door opener in the car died. I was on the phone to a buddy to get him to come over and take me to the store for new ones when I remembered the opener is hard-wired into the house electric. Hit the button by the door instead, dummy!
Not my most shining moment.
I punched myself in the face pretty hilariously (probably!) this summer.
While mowing, I grabbed the left side of the loosely filled paper yard waste bag, leaned over and pushed hard with right on the grass inside to pack the corners and compact the load. The bag ripped without warning and I socked myself with a left fistful of tightly held brown paper. I didn’t fall down or lose a tooth or anything but I knocked my safety glasses askew and took a step back.
What I wouldn’t give for a video, I’ll bet it was really funny. Might as well leave some rakes laying around.
Have done a lot of brake jobs over the last few years. (See photo from my scrap pile.)
So a couple years ago the front brakes on my 2006 BMW Z4 started squealing. Replaced the brake pads on both sides. Very routine. Afterwards, while driving, I heard a grinding noise from the front driver-side brakes. Figured the new pads were settling in. Next day, grinding continued. Next day, same thing. Finally, after a week, I shone a flashlight through the front driver-side wheel. The rotor had deep grooves in it. 
WTF?? I pulled off the wheel and inspected it. I discovered the problem… when I installed the new outer pad, I installed it backwards: the steel backing plate was making contact with the rotor. 
Felt like a complete idiot. I think I am the only person on Earth who has installed a brake pad backwards. Replaced rotor and installed new pads.
This is hilarious! And, just the sort of mistake I’d be capable of making (and I’m a geographer with an interest in aviation, and reasonable familiarity with Paris and with French).
I once changed a tire (wheel) backwards. I realized my mistake (and corrected it) before I drove off, though.
Just had one within the past hour. Computer mouse wasn’t behaving, starting and stopping and not working well, so went and picked up a new one…THEN I googled about it, changed the batteries, and now the original mouse is working fine…ah well, least I have a spare on-hand…
The opener is wired to the house electric but the door sensors (which my opener will not operate without) run on batteries. Oh, and absolutely nowhere in the entire instruction manual, which I only learned of online, is that the door sensors don’t work unless they sense the mechanical vibration of the door opening and closing. Now, on to the true D’oh! moment: after all the years I’d been getting up on a ladder to push the manual reset button on the opener, when I realized that I could have just turned the power switch in the garage off and on.
I punched myself in the mouth once doing a similar thing. Cut my upper lip on my teeth. Owie!
Last Sunday I made meatloaf for dinner. I covered the loaf pan with foil while it was baking. After it was done and had cooled enough to handle I removed the foil, balled it up, and tossed it in the recycling.
Then after dinner I covered the loaf pan with the leftover meatloaf in it with more foil and put it in the fridge. D’oh! I should have just put the foil that was on there when I baked it back on instead of tossing it out. I know it’s a trivial amount in the grand scheme of things, but I feel like I was being wasteful by not reusing the first piece of foil.
I definitely was dazed and remember thinking I could have injured myself if I caught it in the mouth. Then, looking around to see if anyone saw me. Lastly, wondering if an All Powerful God could punch out Himself while packing His yardwaste bags or if the gravity from the infinitely densely packed grass would overtake His Own fist.
The conclusion tests came back unconcussive.
A little while ago I brought home a carton of ice cream. When I went to put it in the freezer I discovered that the only spot to place it meant jamming it in on its side. So I took a whole lot of stuff out of the freezer, rearranged everything on replacing it and was able to put the carton in upright. This meant that the ice cream wouldn’t leak out all over the freezer. It was at this point that I realized that the ice cream never leaks out all over the freezer because it is ice cream not milk. The couple of times since that I have bought ice cream I have just jammed the carton in, as I always had done before my aberration.
Few years ago, I’m starting to mow my yard with my garden tractor. I finish going back and forth in the front yard. I’m ready to do the back, look at what I just did and it occurs to me I never engaged the blade. Just wasted gas mowing nothing.
Another “D’oh” moment is sometimes I try to start my Nissan Sentra by pushing the heater ON-OFF button instead of the START button. Hey, they’re really close together!