I presume you are alluding to the fact that the 787 is certified for a double derate - this means both fixed and assumed temperature method (ATM). On the 787, the engines have been certified at 100%, 90% and 80%. Crews can then add an ATM on top of that. Whilst this saves engine wear, in the event of an engine failure if the crew were using a double derate then pressing TOGA would advance the thrust levers to 100% (or 50% on one engine) and risk having full power on one engine whilst flying at a speed below VMCA. Hopefully that makes sense...
Crews forgetting to retract the gear - whilst possible its not an assumption within Performance A.