A 2 person crew aircraft has one pilot flying (PF) or monitoring and responsible for what the autopilot (to include autothrottle) is doing, and the other pilot monitoring (PM) the actions the PF and aircraft. An approach would typically have have the PM calling out airspeed (and sink rate) at set points based on a Vref, typically at 500' AGl and again at 100' AGL. It is, however, incumbent on the PM to call out any serious deviation from the planned or "targeted" approach speed or a higher or lower than normal sink rate at any time (at my company Vref + 5 is the minimum target airspeed allowed on the approach) along with deviations from glideslope(path) and localizer or course. Going below Vref with a decreasing trend would be very serious and would certainly generate a call. This is really very basic airmanship and speaks to a lack of basic situational awarness on the part of the crew.
The Takeoff Weight entry is more of a problem. Suffice it to say there are a number of cross checks made in an effort to trap a mistake, but if the payload weight is wrong everything else will be wrong.