My bike's speedometer claims that I've been cycling with 87.3 km/h
What happened was that I put the sensor too high, so it was too far from the wheel and couldn't read the magnet. Then I tried lowering it just by pulling the cable, without having to cut the cable ties that were holding it into place, and I ended up breaking the cable. The awkward moment when you're having issues with your bike's electrical parts.
I asked the guy at the repair shop whether I should try soldering it, he told me to bring to him. He just put the cables together and fixed them with some heat-shrink tube. The awkward moment when an electrical engineer is having a bike repair guy fix her electronics.
As for the indication: I had a cable dangling from the display holder and another cable dangling from the sensor, so I coiled them up with some tape and started cycling to the repair shop. At some point, as it was coiled up, it apparently found a magnetic field somewhere and showed this measurement.