This caterpillar looks just like lichen! It doesn't move much. When I checked on it two hours later, it was exactly where I left it. Nestled in a rill in the bark, it was hidden in plain sight. When I touched its back, it quickly flipped its head backwards, as if to bite. Has red spots on underside. Very cool little critter.
Suburban yard, on trunk of dead oak tree.
Note on Bug Guide suggests this might be an unusual color morph: "Per Wagner (2005), most caterpillars are gray and essentially bark-colored, but very occasionally spectacular lichen mimics are encountered." (http://bugguide.net/node/view/24507)