Anything metal in the road that's been polished to a nice mirror like surface by hundreds of cars is brutally slippery in the rain. I feel my bike step out a foot or so on wet mornings when I hit a bridge expansion joint on the Lakeshore/DVD west on ramp. It really is as bad as hitting a patch of black ice.
So watch for street car tracks, man hole covers, catch basin covers, bridge expansion joint or metal plates they put down sometimes in construction zones.