Lighting can also limit you. A low sun glaring into your eyes can make it very difficult to see much of anything, and I frequently end up riding one-handed as I use my left hand to shade my eyes in this situation. A low-hanging sun can also throw shadows across turns. On more than one occasion I have watched a rider dive into a shady corner and emerge sliding on his butt after encountering sand, leaves or some other slippery material hidden in the shade.
Shade can actually create a hazard. The mountains around the Ozarks frequently get a nice coating of dew overnight, and the winding roads that traverse them are often slippery until the sun reaches them and dries the dew. However, during the winter months, the sun may never get high enough to remove the dew from a corner, so the shaded portion remains wet all day. These moist patches will often exactly match the shaded area, making it very difficult to tell that the road is wet unless you are aware of this situation. In colder places, the shaded area may hide black ice, and in fact this dew sometimes freezes on cold nights.
Anyway, whether it's shaded dew, sand, oil or a wild hog, any hazard that you discover on your intended line as the turn reveals itself requires an alternate plan. If you have entered the corner at a reduced speed, you should have enough reserve traction to do some braking, even though you are leaned over. The slower you are going, the more options you have, including perhaps stopping. And if your only option ends up being an off-road excursion, reducing your speed means you will do less damage to the guardrail or tree that you hit.
If you are going slow enough, you can simply straighten up and ride across something slippery. If the hazard is in the middle of the lane, with some reserve in hand you can go around it. I prefer to tighten my line and go inside, because that allows me more options for the rest of the turn if there are further hazards. Also, if I misjudged, I can straighten up and still have pavement left to use for further slowing and changing my line. Going around a hazard on the outside often leaves you leaning over close to the edge of the road, so there is no room to straighten up and remain on the road if things don't go according to plan.
Fortunately, there are plenty of brightly lit, perfectly clean corners that you can see all the way through before you have to commit to a line and speed. Those are what make all the dirty, slimy, frog-infested ones worthwhile.