Some audience members wondered why the car in shot in the scene outside the present day flat had no rain on it. It was actually raining when they shot the scene. The reason you can't see any rain on the car is the car wasn't there as it was unavailable so it was added in post. Because the rain didn't show up well on screen a rain effect was also added in post.
The first day of shooting was at Motel 22. The second at the lab location that had a 17 spray painted on a roller door. So the crew decided to put numbers somewhere on the set of every location. As the shoot lasted over a year this was latter dropped or forgotten about but lots of sets still have numbers hidden in them.
The alley cat, Captain Bung-eye, was a rescued cat from the SPCA. There was a year gap between shooting the scene of him in the laboratory to the scene of him in the alley way, during which time he put on a lot of weight. It was suggested that as well as a time machine Matt had invented a new weight loss system