A simple little situation that surprised me when I first discovered it. It comes up often in tripod group variations.
Thanks to Bommel for finding the correct correct solution.
(;W[or];B[os]C[Black either escapes along the bottom, or gets a ko. It's a hard ko, to be sure, and a difficult crawl, but white can prevent both.])
(;W[nr];B[rr];W[ns]TR[ps]C[Black can't play at Q1, and if he captures, white will play there to make a false eye. Good job!RIGHT])