During the Timothy Leary scene, the kitten that Chong is holding disappears and never appears again without explanation.
Donna tells the Highway Patrol officer that her apartment number is 714, but when Animal comes to the room, he knocks on 716.
When Donna's 'old man' Animal steps into the glass elevator, appropriately, none of buttons are illuminated. A moment later, he reaches for the button panel, but the light is already on for the 7th floor before he presses it. The scene cuts away for a few moments, and the next medium shot of him in the elevator now shows the 5th floor button lit, not the 7th, which should still be on.
(at around 33 mins) When Cheech is trying to start the ice cream truck, someone's arm becomes visible, though nobody is shown before or after this scene.