In the scene where Alan cuts himself with a razor blade, they scan through his apartment. On the coffee table was a commercially made plastic bong. The first plastic bong was patented in 1975, six years after the time line of the movie. Prior to the mid 1970's bongs were made of bamboo.
Alan's "inner voice" says, "You are not going to Toronto with Cheech & Chong here while your mom's lying in a hospital...." The story is set in 1969, when Cheech & Chong were still unknown comics working in Canada. Their first album wouldn't come out till 1972.
In the scene where Alan waits for the train to go past, there are "double stack" inter-modal cars on the train as well as an "End-of-Train-Unit" or "SBU" (Sense and Braking Unit, which replaced the cabooses). Both of these things came into use in the 1980s and this movie is supposed to take place in 1969.
Alan hides in a refrigerator after being chased by some rednecks. The rednecks shoot at the refrigerator where he is hiding, but when he emerges from the refrigerator, there is no evidence of the door being shot at, either inside or out.
The rednecks follow Alan for a long distance while leaving their car behind. When Alan hides in the refrigerator after a short while you hear the rednecks leaving in their car (which they actually left behind a long distance behind).
As Alan Parker is heading to Lewiston, ME, he is seen walking past a sign indicating the junction of US Routes 2 & 68. US Route 68 runs for 560 miles between Reidland, KY and Findlay, OH. The closest it comes to Lewiston is 714 miles.