I came off this morning* on a road I ride every day, in all weather conditions. Last night was frosty so the road was salted/gritted, but there was no visible frost nearby, it had warmed up from the overnight minimum, and the road is quite heavily used, both warming it and grinding the salt into the surface. But the road was damp.
So why would a salty, damp road be worse than the same road just damp?
More info, just in case it's of interest:
I was only doing about 20km/h and I've taken that bend faster in the wet before, on the same tyres when running late for my train. I was following a racing line (outside-inside-outside) so although it's a fairly tight bend my line wasn't tight.
I don't believe I just hit an oily patch. It felt slippery underfoot (walkable SPDs) when I picked myself up having slid a little way. Also I take pretty much the same line every day.
It was about 8.9 km into this ride.
* I'm OK, and so's the bike.