In the first two mini-series, the Visitors spoke with an electronically modulated voice. The sound processing, however, was dropped for the ongoing series. This created a continuity problem with the two mini-series as the Visitors were able to infiltrate human strongholds and vice-versa without their different voices giving them away.
The trivia items below may give away important plot points.
The script of beginning of the second season was completed while the first season was still in production. After the series was canceled, it eventually got leaked. Most notable was the fact that Julie was killed during the first act.
Because the series was canceled, the producers had no way to finish the story because the first season ended with a cliffhanger. Interest in the ongoing comic series had also waned, and it was canceled as well. Thanks to the warning the comic-book writers had received, they were able to write a 2-part story that served as the official ending for both the TV series and the comic-book series: the Leader sends his only child to Earth, who is befriended by the resistance. In spite of the war-minded Visitors plotting an assassination attempt to frame the humans, he was returned safely to the leader, and as a gesture of thanks, he recalled the attacking fleet from Earth.
When the romance between Mike and Julie began to fade, both Faye Grant and Michael Ironside pushed to have a romance between Julie and Ham. Unfortunately, Ham was written out before this could happen, and the show was canceled before he could return.