Real heating jobs itemize: climb the slope with $Q = mc\,\Delta T$, cross the plateau with $Q = mL$, and add the legs. The named trap is skipping a leg — usually the plateau, because the thermometer stops advertising it.

And feel the sizes before computing. Heating a kilogram of water from ice-cold to boiling costs $4.2 \times 100 = 420\ \mathrm{kJ}$. Boiling that same kilogram away costs 2260 kJ — more than five times the entire climb. That is why a kettle reaches a boil in minutes but takes its sweet time boiling dry, and it is why the estimate-first habit from kinematics rides along here: one glance at the data sheet usually tells you which leg owns the bill.