Trap One: the Corner That Needs Two Points
One-point perspective only works when something faces you square-on. But turn that box so a corner points at you — the way most things in real life actually sit — and one vanishing point can't cope. Now you've got two sets of receding edges going two different ways, so you need two vanishing points, both sitting on the same horizon, one off to the left and one off to the right. The left-hand face aims at the left point, the right-hand face aims at the right point, and the verticals stay perfectly upright. Flip to "one point" in the toy and the box looks like it's melting; flip to "two points" and it snaps into something you'd believe.
Trap Two: Forgetting Where Eye Level Is
The horizon is your eye level, and that has a consequence people forget: anything above the horizon, you see the underside of (look up at a high shelf — you see beneath it); anything below the horizon, you see the top of (look down at a box on the floor — you see its lid). Drag the box up past the horizon in the toy and watch its top face disappear and its underside swing into view. Draw a building showing its roof and its underside at once and the eye instantly knows something's off.
The Quiet One: Over-stretching
Push a vanishing point too close to your object and the box bloats and distorts, like a photo taken with your phone an inch from someone's nose. Spread the vanishing points wide — even off the edge of the page — and everything calms down and looks natural.