The last row in F, on those planes that don't have a floor-to-ceiling bulkhead divider, but just a partial bulkhead and/or curtain -- sometimes the passenger in the middle seat of the first Economy row will put his (never her) feet up on the back of the F middle armrest. One time, during takeoff, when the plane was at a sharp angle, my water (PDB) spilled down the armrest and got the passenger's socks all wet. Problem solved.