[greymule]

Let's say the team on offense has been aggressive base runners the whole game; they have managed to stretch a walk earlier into a "double", executed a delayed steal, and in general are risk-takers on the bases. Catcher throws hard back to F1 after the walk to hold the runners, but the throw is a bit high and the ball is tipped off F1's glove back toward 2B. R1 sees this and takes off. F4 backs up the throw, gloves the ball, and R1 collides with her. Just to be clear, F4 has possession of the ball several steps before the collision and the collision happens between 1B and 2B. F4 goes down, but R1 stays on her feet and advances to 3B, beating the throw from the recovered F4.

That one has to be interference, and the runner should be declared out even though the defense cannot put her out.