I may have unjustly disparaged the M50 bus when I wrote about its poor performance earlier: http://jeremymassler.blogspot.com/2011/05/m50-cross-town-bus-runs-on-my-street.html
There are a couple different buses that I see travelling down 50th and 49th streets on a regular basis. Some of them are clearly charter buses, and some are MTA buses with a "FERRY" sign on the front. One such "Ferry" bus was fully adorned with an advertisement for "Stomp," while others look pretty much like normal buses, and it was doubtlessly this type of "Ferry" bus I saw on the heels of the regular M50 bus.
So my expectations of the MTA bus system have risen from "quite low" to just "selectively low." I know for certain the M35 bus once experienced a "lap" when I got off at Ward's Island (after having waited 20 minutes too long at the normal departure point), looked back behind and saw the trailing M35 bus sitting 50 yards away.