Lots of comments about lightning, but I believe you are referring to a "ground loop" where the ground/neutral at one site is offset from that of a different site tens or hundreds of meters away. This is why all commonly used RJ-45 Ethernet connections use differential signalling and have the tx/rx pairs isolated by transformers. (Note that PoE Ethernet can still create similar problems if one is not careful.)
Here's a relevant news article about a lawsuit related to this phenomenon: