Tying a noose. At Basic Training I got tasked to help the Drill Sergeants create their haunted house (Battalion competition... long story). My Drill asked if anyone knew how to tie a noose, and I whipped out a perfect one in only a few seconds. He just looked at me and said "Private. What the fuck."
I once had a neighbor catch me while I was taking groceries out of my car and ask if I knew how to open locked doors as they locked themselves out by mistake. Went over to their door, took out a credit card, and shimmied my way in in no more than 5 seconds.
Probably was bittersweet for them.
Being able to quickly convert between Ounce and Gram
Remembering things about a person from a few months/years ago when the person herself can't remember about it. You just look like a stalker and it's frustrating
Genuine kindness. My father was such a good soul, and people were suspicious about his kindness sometimes.
The removal of stains.
Eavesdropping. I just passively listen to other people a lot when I'm alone, even if I'm doing something else. It gets really embarrassing when you bring up information you heard from their conversation forgetting that you weren't involved, or when you laugh at a joke said by someone across the room.