These types of errors happen more than we would know I suspect. Most get picked up b4 power up. Humans make mistakes & we all know that fatigue (not saying it is the case here), high levels of automation & the saving of $$$ all adds up to a risk level that's getting higher.
Flex temp T/Off (reduced T/off thrust) is purely to save the engines with less wear & tear as we operate out of Rwy's longer than actually needed but that comes at a cost as we have seen here in this latest J* event.
The basic fix for this scenario is go max thrust on every T/off but that costs $$$$, something that aviation now revolves round more than safety!
I do a gross error check on the weights of the 'bus' in my head b4 I/we launch, old school I guess. Eg. 43 tonne Op Wt machine (Fleet average rounded up). 8 tonnes gas (typical 2 hr sector). 180 seats (A320) I round that up to 16 tonnes for a full ship(allows for average bags under floor also).That's 67 Tonnes as we trundle out to the black stuff (Rwy). The actual current GW of the 'Bus' is always displayed on the bottom R/H side of the SD screen when the plane is 'alive' so I compare that to what I worked out in my head, it's always within a tonne or so often a few 100 Kg's, not enuf diff to effect the T/Off drastically. Obviously the exact figures have been worked & entered but there in lies the possible error as is the subject of this discussion.
It will happen again you can count on it !
Wmk2