The closest I have come was just on this last Monday evening. (Yes I know normally We wait a few years till we talk about our stuff ups but I'm not normal(-:)
I had been out for a late arvo sunset flight (my favourite flying time) and had a passenger to drop off at a nearby farm. The farm strip (hardly a strip, it was a farm track with a gully one side and a corn crop on the other with washouts here and there) was one I had landed on before and all went well and I landed without any trouble and dropped off my pax. Now this was the first time with this pax and I had supervised his loading in but didn't watch him getting out which was the first link in the event. When he got out his trousers or leg must have bumped my park brake lever just enough which didn't affect my takeoff as I had lined up before he got out so I didn't use the brakes at all but it was when I got home that things woke me up!
As part of my prelanding checks I squeeze my brake pedals to make sure I have pressure and this unwittingly locked my brakes on. (I will now be adding check park brake off to my list, I should've already done that but will explain that later) and the road I was using at home had a nice 5knot quartering crosswind but it was a fairly constant wind so nothing taxing and I had had a beautiful flight and was quite comfy with how it was all going that was until I touched down, upon touchdown of my upwind wheel it skidded causing the nose to swing towards the wind bringing down my other mainwheel and lifting my tail up to about level. It was an instant thing to bump power on to get some rudder and elevator authority but it was so so close and was enough to give my heart a good speed up.
I was lucky that a few things were going my way, for starters once I had weight on the mains they spun as the brakes don't quite have the Oomf to lock them solid with the big wheels, and secondly always being taught to have the hand on the power in the circuit was totally essential, a split second of delay and I would've been around and had a bent wing at the least.
So I was really the cause (isn't a ground loop always initiated by the pilot!) and I should've known better. Why? Well a month or so ago I took forumite old K for a local flight to scout some local strips and for any here that don't know him he has long legs and on top of that when we went I had the pax seat one hole forward so his legs were a bit far forward. Well in the time it took us to fly to the first strip his gangly legs had pulled the same park brake lever on. The landing was uphill and being a new strip I was planning on braking hard anyway so we did a nice short landing and I didn't notice anything wrong until I went to taxi, well it was on a little slope but I needed full power to move. I had my belt off to jump out and check things over before I spotted what had happened. So how's that for a classic human factors stuff up. I had knowledge of this happening but still didn't put enough emphasis on it to add it too my checklist as I justified it to myself as being a once off occurrence due to old ks long legs and incorrect seating position, not thinking that someone getting out a bit differently could do exactly the same thing.
On top of putting it on my checklist I'm also now thinking of adding a lock to the park rake to totally rule out a similar problem. Anyway it's still fresh in my mind but is the only time I've ever felt close to ground looping so thought I would share.