Takeoff data entry into the FMS is one of the most critical stages of flight which requires the upmost diligence as the consequences can be catastrophic. The routine of turn arounds, time pressures and complacency are major threats to this duty, and mistakes and wrong data entry can be easy to do.
But as mentioned earlier, cross checking the load sheet T/O weight with the FMGS calculated takeoff weight after ZFW insertion should be a fail safe, unless of course the is an error with the load sheet.