Continuity mistake: When Monty drives Withnail & I into Penrith the radish is on his left lapel. When they arrive in Penrith a few minutes later, the radish has mysteriously moved to his right lapel.
Factual error: When they are driving back home from the holiday cottage you can clearly see an M25 sign in the background. The M25 didn't exist in the 60s.
Continuity mistake: When driving to Penrith, there is a shot of the road taken from the driving seat of the car. There are two headlight patterns on the road, but the car only has one headlight. (00:33:00)
Continuity mistake: Whenever you get a shot of the car travelling along the motorway or road, the cars going in the same direction as the Jaguar are all true to 1969 - the year the film is set - but the cars travelling in the opposite direction or parked by roads are all contemporary with the time it was made (eg. Ford Sierras).
Continuity mistake: In the breakfast scene with uncle Monty, he is holding Withnail and I's hands. They both let go and stand up, and as the shot goes behind Monty's head you can see that he is still holding I's hand.
Continuity mistake: When the car leaves the motorway on the sliproad as Withnail & I first head out into the countryside, there is only one person visible in the front of the car. Withnail said he was going for a sleep, but it would have been too difficult for him to make it into the back of the car whilst it was moving. (00:32:10)
Factual error: The roadsigns on the motorway are quite contemporary (1980's), the signs used in the 1960's would have looked considerably different. To travel to Penrith, Withnail & I would have had to leave London via the A1, and not the M25 motorway, which wasn't constructed until many years later.
Continuity mistake: When they are driving to the country there is a blue Ford Escort driving behind them which is also driving behind them on the way back.
