Plot hole: In the episode where the Sliders go to the world where Quinn sees himself as a child attending his father's funeral. Quinn helps his young self deal with bullies. Quinn repeatedly mentions is was all happening the way it did on Earth Prime. The flaw here is that in the Series Premier episode, they showed a picture of Quinn, his mom and dad. Quinn was a teenager in that picture, not a kid. Therefore the events in the alternate world they were visiting could not be happening the exact same way. The man in the picture is the same man shown as his father in the other worlds so it cannot be a "stepfather" that his mom remarried.
Plot hole: Xena is posing as a guard outside of Caesar's office. She knocks out the 2nd guard and throws him on the ground. Moments later, about 10 guards walk by, but no one notices the passed-out guard on the floor. (00:14:40)
Plot hole: WARNING SPOILER AHEAD. The alien impersonating Skokes changed the ship's destination to Earth. This is impossible to have been done, as the first order the captain gave was to lock the course in and "Skokes" admits his security clearance was removed after his rescue from his prison. It also couldn't have been done by another alien spy, as no other survivors beyond the 4 (2 human and 2 aliens) were found and the only other alien spy was unconscious, so he couldn't have helped out in any way.