Black ice of the most vicious kind, just leaving a parking, and turning onto a little trafficked road in an industrial area.
Was driving my uncles early 90s Wrangler and as I turned onto the road reasonably gingerly, the back end just came round. Wasn't even making any kind of spirited get-away, just lost it.
I think I gave it some oppo, and slammed on the brakes, because I was just heading for the curb in slow-mo, but drove home in the most pussy-footed manner imaginable, never exceeding 20 mph. That annoyed some other drivers on the road, but I didn't feel like pushing my luck. Hitting that kind of ice before a red light or going into a corner at faster than walking pace, and the Jeep would become a ballistic death trap. I also did engage the diff-locked 4x4 after that, so that at least in corners I might get some grip to haul me around.
Also had ABS engage once, while rolling to a stop on snow/ice.
Literally I was doing 3 mph or so, lightly touched the pedal, and all it did was vibrate back at me. First year of driving, LWB Chrysler Voyager/Caravan on summer tires in winter.
And the Citroen C6 on the way home from my grand-parents place, wit a surprise snow. Also all-weathers/summer tires, and snowy streets. Had quite some losses of traction and rallyed that thing through some of the snow.
And finally I was ballast on the rear axle of an empty white van once, going uphill on a patch of ice. Without ballast, it would just spin the tires, but with me and a buddy sitting on the sills out back, it got over the crest.