Which way round, and what a wrong seat joins

The mark is the whole check

The shield's silkscreen says the Nano fits in only one direction, and that turning it the other way can permanently damage the board. The sockets do not back that up. They are two rows of 15 holes, 15.24 mm apart, exactly where the Nano's two rows of pins are, and the arrangement is symmetrical: a Nano turned through 180 degrees fits as readily as one the right way round.

It cannot be flipped over, because the pins are only on its underside, so turned round is the one wrong way that goes in easily. That is the way the mark is for.

Look for three things at once. A boxed USB at one end of the shield. An arrow beside it that reads "USB IS ORIENTED THIS WAY". And on the Nano, the metal USB-C socket, which goes at that end. To check it by names rather than by a mark, find VIN on the Nano and VIN on the shield: they are at the same end, away from USB.

What each wrong seat joins

The pin names are different at the two ends of every wrong seat, so every pin lands somewhere. The figure draws the Nano pin that lands beside each hole.

Turned round. The Nano's row A lands on the left socket, backwards, and its row B on the right one. No pin meets its own name. Ten put a power or ground pin on a signal hole, or a signal on a rail: the Nano's VIN meets the shield's D12, its 5V meets D9, and its 3V3 meets RX. Another twenty cross one signal onto a different one.

One hole along, or one hole back. The sockets are exactly 15 holes, so the Nano's last pin on that side has nothing to land in and every other pin meets its neighbour. One of these puts VIN on a GND hole, which is a short between a rail and ground, and seven more put a rail or ground on a signal hole.

Rocked up at one end. One end clears its holes first, and the pins that have let go meet nothing while the rest still touch. Pulling a Nano out with the power on does this slowly, one pin at a time.

What a join like that can cost

A pin that is an output, joined to a rail, is an output asked to drive straight into a supply, and the chip's limit is 40 mA absolute on one pin. A rail that meets a signal hole is a supply driving the input of one of the shield's inverter chips. When a chip's supply pin has lost contact while its input pins are still driven, it can be powered through its input clamp diodes, which TI rates at 20 mA. In each case the word is "can": it depends on which pins, and on what is plugged in at the time.

So the figure never shows a temperature. What the owner's bench says is that the shield may get hot. That is the cost of getting the direction wrong with the power on, which is why the power goes on last.