When Two Worlds Collide - S5-E3
Continuity mistake: Gary has to feel around in mid air for the portal as the wall has just been bombed. But when he reaches through, later, his arm comes out of the wall which has somehow returned.
Goodnight Sweetheart (1993)
1 mistake in season 5 - chronological order
Ron Wheatcroft: How do you know the future you've got now is the same future you're going to have if something in the past is different from the last time you went back to the future?