[greymule]

R1 on 2B. R2 on 1B. BR hits to shallow RF. All runners off, but R1 stumbles and falls (no OBS) before reaching 3B. R2 stops after rounding 2B waiting for R1 to get going again. BR rounds 1B and is heading to 2B when F4 takes the cutoff and tags BR. F4 throws to F5 at third who tags the base just before R1 finally touches 3B.

Is R1 out?

I've actually seen this type of situation called a force out. In a HS game, R1 on 1B, BR got an extra-base hit, and with his speed had actually passed 3B before R1 reached home. So because there were two runners between 3B and home, the ump rules that R1 had become "forced" at home and could be put out with a mere tag of the plate.