This is a basic shape which often appears in real games and which surprises most beginners the first time they have to deal with it.
(;SZAW[ho]AW[hp]AW[ip]AW[iq]AW[ir]AW[hs]AB[go]AB[gp]AB[hq]AB[hr]TR[hs]C[White has just played at the marked stone. What is the correct response? (Think late endgame/yose--Black is trying to seal off the lower left side.)]GMFFCA[UTF-8]AP[CGoban:2]STRU[Japanese]KM[6.50]TMPW[White]PB[Black]
(;B[er]LB[fq:B]LB[fr:A]C[Good, but playing at A might be slightly better (there might be some bad aji left at B).RIGHT])
(;B[fq]LB[er:B]LB[fr:A]LB[es:C]C[It's ok, but the last stone is usually played at A or B (as it removes bad aji from C).RIGHT]))
(;B[fr]C[No need to push further.]))
(;B[is];W[gq]C[This is rather catastrophical for black. You could have avoided this.])
(;B[gq];W[fs]C[You could have avoided this.])
(;B[fr];W[fs]C[You can do better than this.]))