The van is facing away from the guillotine. The professor drives it hard forward to eject the bad guy out of the back and then halts, but then suddenly the van is somehow facing the guillotine when the professor drives away again.
When the sliders escape the execution, they are landing in the Little Green Men World, but the person with the camera is clearly shown to be neither.
The ventilation shaft entrances are high on the walls - but when Wade and the female detective jump out of one into the corridor, they are clearly only jumping from a height of 1 or 2 feet onto the floor.
Taryn looks up Quinn Mallory's file to discover he has a scar. When she talks to Quinn, she reveals information about the scar that wasn't in the file.
Quinn says there's ten seconds left until he has to activate the timer, but about twenty seconds pass before he does this.