Continuity mistake: When Fletch goes to visit Calculus in his shack, he knocks on the window and sees Calculus sitting in a chair, wearing boots. The next shot, as Fletch is walking inside, Calculus is now wearing socks and no boots, despite never having left the seated position, nor having had any time to slide his feet out of the boots. (00:35:55)
Continuity mistake: When Fletch goes into the Biker Bar it's pouring rain, but when he comes out minutes later and goes riding on a cycle, it's not raining and the ground is completely dry. (01:09:05)