This one is entirely on me, and in retrospect its hilarious.
I've done 3 big projects on the house this year, totaling over $15k. All planned, budgeted for, etc. No issues there.
I've also done various smaller things, supplies for DIY, repairs, etc. Combo of repair/maintenance and improvements, but this is the smaller stuff. Also budgeted for, not necessarily planned (don't get to plan to have the faucet break!). No issues there. Total - around $5k. (old house, new owner, and previous owner didn't necessarily do all the maintenance. Plus, their landscaping was crap)
I use Quicken to track my expenses. A little earlier this year, I'd decided I didn't like how I was categorizing the house related stuff, so I changed it. Basically, I'd broken it into 5 categories, and I changed it so that they're now sub categories which roll up to 1.
Today, I was reviewing my expenses, and happened to somehow see a report that wasn't broken into the sub categories, but instead only showed the top level one. Scared the crap out of myself when I saw a category that was around $20k. Took a moment before I realized that it was just adding together the different house sections. Yes, I really do think about this stuff that way.
MPP: I changed categorizing and forgot that I did it, then scared the crap out of myself.