This is an interesting one. De Bruyne lofts a ball into Sané’s path. He is clearly not offside.
Karius punches the ball, but only manages to clear it 6 yards. It hits Milner and bounces back toward Sané, who is in an offside position when the ball hits Milner.
Many people do not understand that Sané is in an offside position at this point because the goalkeeper is actually the second-last defender in this case.
However – here’s another thing many people do not understand – Sané is not committing an offside offence because the ball came off Milner. Being in an offside position is not the same as committing an offside offence. Law 11 clearly states offside is determined by when the ball is played a teammate, so the relevant instant of play is actually a few seconds earlier when De Bruyne initially lofts the ball toward Sané, and he is not in an offside position then.