For example everything breaking and electrical arcs from outlets. In the early days of the telegraph we did not understand ground currents or the difference in ground potential over geographic distances, and it would occasionally cause large, lethal discharges from the wires. This would occur with zero warning.

Ground current means that there is a potential difference between two points. Over large distances a small current will lead to extremely large voltage differences. Wires over the same distance will have very low voltage differences (since they are low resistance), so the wire will be the same voltage as the place it was last referenced to ground. That may be very different from the ground or people at a different location.

This can be particularly important in improperly wired homes, which can have small voltages on a floating ground. Ground currents will cause that voltage to become much larger.

> The James Bay network went offline in less than 90 seconds, giving Quebec its second massive power outage in 11 months.[10] The power failure lasted nine hours and forced the company to implement various mitigation strategies, including raising the trip level, installing series compensation on ultra high voltage lines and upgrading various monitoring and operational procedures. Other utilities in North America and Northern Europe and elsewhere implemented programs to reduce the risks associated with geomagnetically induced currents (GICs).[9]

https://en.wikipedia.org/wiki/March_1989_geomagnetic_storm