Worst one in recent memory was 14 degrees. It was prime conditions for ice, so I pulled the bike out with studded tires, headed out at Allaire, and noticed after ~.5mile that the freewheeling was lazy. I pressed on--because it's not like you coast at Allaire that much--until the ~2mile mark. Going down the hill behind the care center, headed to the park proper, I got kicked forward by the pedals.
As a consummate fixed gear rider, brain just went along with it , so I didn't crash, but once I stopped, I realized that the freehub was no longer working properly. I could, with great force, turn the pedals backwards, and it would drive forwards, but I realized that something was really wrong. Not wanting to get stuck out in the (rapidly dropping) temperature, I booked back to the car.
As it turns out, the oil that I'[shouldn't]d lubed the freehub with froze, and the unexpected resistance detonated most of the parts of the drive mechanism...but the drive plates still hit, so I managed to self rescue. It would've been a really freaking cold mile back to the car.
Absolute coldest was years ago in Trenton, probably around that by a few degrees. Like
@Fire Lord Jim I decided that it wasn't "fun". I have better gear now, though.