I've heard stories of unforgettable moments, on the way to an event, at the event, or on the way home.

I drove 560 miles to spend the weekend at a dry lakebed and work on a few ideas and to play. One reason for the trip was to work on an improvement on the blokart. I had everything needed for the test and looking forward to a great time of trial and error.
Somehow I left the base of the mast at home. With no way of making anything work to replace the mast I had a long weekend of wishing I would have ...........
On another trip I grabbed one of my 2 red duffel bags that contained my harness, helmet, AQR, and line sets. Only to open it up at the beach to find it contained other things for land sailing and nothing for the kites. I turned around and went home and spent the weekend making large orange rages for kites and blue rages for the blokarts and yellow for the land boards.

Montana snowkite rodeo.
We all went up to Mount Hagen. We got there and I discovered I left my ski boots in the hotel. Missed an EPIC adventure.

I took a friend to the beach for a 2-day kite buggy adventure on my gear. As we unpacked equipment on day one, I realized that the perfectly-sized kite for the day was sitting at home in the garage. It was the only equipment I forgot.

I've heard of buggies being lost off the back of the car.
A buggy that was hanging too low off the back of a motor home as it left the gas station.
And a buggy that was crushed as the car it was strapped to the hitch as the car backed into a parking space.
Any others?

I was headed to a weekend trip at Jekyll Island and a little over an hour into the trip, while going through mental checklists, realized I had left the front end of the buggy sitting in the garage. The wife was not happy when I said we had to turn around. 2.5 hour delay on a 4 hour drive.

This is a good thread! I don't have large examples but early on in my power kiting adventures I would show up at a local soccer field without my harness, no bar and lines, and other silly things like that. To overcome these sorts of things I have for a while now organized my kit onto just two shelves in my garage and have consolidated all my kite extras (harness, body armor, etc) into two bags. This way I just have to get my buggy into the back of my SUV and clear my two garage shelves entirely to know I have all my stuff. This still leaves unusual items like my pink and gray tubes for securing kites on the playa but otherwise (fingers and toes crossed) I've got this nailed down.

I ALMOST forgot my rear axle and wheels once just like Bobby mentioned with the front end. Doh!

I've left things behind at home (and a few times left things behind after I was done) but nothing really big. However, what's worse than is I spend lots of time looking for stuff that is in my car (e.g. the perfect kite for the day, camera, GPS, line, handles or bar), eventually I give up and use something else. Usually later on I found that it was in my car all along.......If you can't find it - it is no better than leaving it at home.

I have only forgotten my harness for a quick evening flight in my hometown.

The kite I had brought was in the peak backpack harness. I figured I would give it a try. The lower leg straps are made from elastic. It basically became a big stretchy wedgie machine and the attachment point ended up near my chest. I stopped flying quickly and decided to go home.

No replacement for the proper equipment.

Drove 8 hours to Alvord with my family in the car and the RV in tow. Only to discover that my buggy had been damaged and I did NOT have the necessary gear to make a repair. Only saving grace was that I had a second buggy with me. Unfortunately it was set up for my son. Not me. Still had a great time but did not accomplish my goals for the trip.

couple times i forgot to take my coffee outta the car...
once i left my beers at home...