The method from rung 2 is solid. The trouble is people reach for “add” out of habit, even when they're hunting a shorter side — and that quietly gives a wrong, too-big answer.
Trap One: Subtracting, Not Adding
If you're finding a shorter side, you must subtract the squares: a² = c² − b². Add them by mistake and you'll get a number bigger than the hypotenuse — which is impossible, because the hypotenuse is the longest side. The fix is a one-second check: “is the side I want the hypotenuse?” If no, subtract. In the toy, choosing “add” on a shorter-side question stops you and explains it.
Trap Two: Which Side Is the Hypotenuse
You can only call it correctly if you spot the hypotenuse, and it's always the side opposite the right angle — the slanted one, the longest one. In a worded problem it's the ladder, the diagonal, the straight-line shortcut. Mislabel it and both the add/subtract choice and the whole sum go wrong from the start.
Trap Three: Rounding and Units
Two finishing-line slips. First, keep the units the same — don't mix metres and centimetres in one sum, or square one before converting. Second, round only at the end, to one decimal place, and carry the unit through: √52 ≈ 7.2 cm, not just “7.2”. Round halfway and small errors snowball.