Plugging in a flight battery/LiPo backwards (reverse polarity) will irreversibly damage electronics such as a flight controller (FC), electronic speed controller (ESC), and/or all-in-one board (AIO). Such damage due to user error is not covered under warranty. All warranty claims found to be the result of reverse polarity will result in labor, parts, and shipping costs for the user.
Reverse polarity issues are more likely to occur with smaller battery connectors, such as XT30 connectors (e.g. BabyHawk R series drones) and PH2.0 connectors (e.g. TinyHawk series drones). This is because the ratio of tolerances to dimensions is necessarily higher, and the material thicknesses are necessarily thinner. An example of plugging in a PH2.0 connector with reverse polarity: