100 ton take off weight input error?
Now there's a safety device they could easily design into the system. The computers know how much thrust the engines are set for, the time the thrust's been on, and the speed achieved. Acceleration=Force/Mass, a simple task for the computer to compare expected speed against actual and alert the pilots to the discrepancy.
For gross weight errors this could probably be detected during taxi thrust, certainly in the first few seconds of the take-off roll.