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: The length of time the vortex stays open varies throughout the show and seems based on when the writers want it to close. In several episodes the vortex is said to stay open for 60 seconds. Sometimes the group go through immediately (to avoid danger) and sometimes wait (to say good-bye). Yet it will still close after the last Slider goes through, even if it's open for less than 15 seconds. And it's not closing because the 4th person went through since sometimes more than 4 people go through.
Gillian Of The Spirits - S2-E3
Plot hole: Quinn is unable to touch anything or anyone, yet he is somehow able to sit in a taxi cab. Why doesn't he fall through the car seat?
Suggested correction: This is a common trope with ghosts or people on the astral plane or out of phase. It's not a mistake since it's not a real life thing and it's not explained how it works (although there are some theoretical physics theories behind it). He can sit in the cab the same way he can walk on the ground and up stairs.
Post Traumatic Slide Syndrome - S2-E8
Plot hole: Since Quinn's double built a sliding system in his basement, our sliders could have used it to reset the timer and go home, just like they intended to do in the episode "Eggheads."
Plot hole: If Quinn has the timer that could be used multiple times without trouble, why couldn't he take back Remmy and Wade then come back and save the world in peril?
The Other Slide Of Darkness - S3-E21
Plot hole: Quinn once again meets the alt-Quinn that he met in the pilot episode. Alt-Quinn tells our Quinn that he remembers solving the final part of the equation on his chalkboard. The problem is, there are many...possibly an infinite number of Quinn doubles in the multiverse. How could alt-Quinn have possibly known he was talking to our Quinn?