Before take-off, pilots enter a number of figures into the flight computers. Among them, they enter:
- The total weight of the plane
- Adjusted thrust
No sense using full thrust on a lightly loaded plane (which would get you a very short take-off) when you could reduce thrust a bit even if that means using a longer part of the runway.
As has been shown in the case of Emirates Flight 407, a single error can then lead to nearly-catastrophic consequences. Incorrect data were entered in the Electronic Flight Bag*, and the takeoff parameters were then entered into the flight computer. As the plane was a lot heavier than the figure entered, the thrust wasn't enough to get the aircraft off the ground before the end of the runway (actually 148 m beyond that!), and hit quite a few things while very slowly gaining altitude after that.
* In this case it was the "Airbus Less Paper Cockpit (LPC) system. The LPC system replaced the majority of the aircraft's operating documentation with a laptop computer-based system." (preliminary investigation PDF)
Now, the error there was massive: 262.9 tonnes instead of 362.9, a full 100 tonnes!
A similar incident happened with AF 6724, with input of 243 tonnes instead of 343. Again a full 100 tonnes!
The error in the case of the TUI flight was 1200 kg (1.2 tonnes). As explained in the report, this resulted in a very minor difference in all parameters. Note that a 737 is much lighter than the 340-500 or the 777 in the examples above (take-off weight was around 65 tonnes), so it's not 100 times less in proportion, but it's still significantly less.
However most aircraft incidents are not due to a single error, but an accumulation of errors large and small that finally end up tipping things over, as every single error removes a bit of a safety margin.
In this case, the report tell us that as a result of the error, thrust was set to 88.3% N1 instead of 88.9% N1. It was still slightly above the minimum for safe take off (88.2% N1), but as you can see, safety margins have been significantly reduced!
Note that in any case passenger weight estimates are just that, estimates. They're also often a relatively minor part of the total weight when compared to the aircraft itself and fuel. But they're still important enough that they need to be carefully considered, and have been readjusted over time to reflect changing patterns.