Relatively speaking the error was minor as 1200kgs would be less than 2% of the MTOW, though obviously it shouldn't have happened and procedures need to be put in place to ensure it doesn't happen again.
Narrow bodies are much more forgiving of incorrect take off calculations than wide bodies, particularly when operating at airports designed to accommodate the larger variants, though a South American cargo B727 was lost when all the holes in the Swiss cheese lined up.
https://en.wikipedia.org/wiki/Aerosucre_Flight_157
Wide bodies are a different ball game with numerous accidents and near disasters happening because of incorrect take off data, the recent SAA A340 comes to mind.