When Calvin is talking to his brother, there are two other people at the right side training on the stepper. After the camera swaps, the two people from the stepper are now at the treadmill.
As Calvin writes the last page of the book, he writes "She was no longer Calvin's creation." in a new line. The next time we see the page in the typewriter, the line starts with "no longer...".
When Calvin and Ruby are having a conversation by the pool, Calvin is eating a burger; during the last shot the burger is intact while in the previous ones it is seen semi-eaten.
When Calvin is telling his psychiatrist the story he started to write, you can see his shirt sleeves folded in different ways, depending on where the camera is.
In their way to Big Sur, when they stop a while near the sea and leave the car, Ruby is carrying a purse. Immediately later, when they are playing "who kills the other first" you can see no purse at all.
When continuing to write on a page with the typewriter, the main character inserts the page the wrong way.
When Calvin drives his BMW home from the party at night, the tachometer shows the engine at 0 rpm, and the needle never moves.
The zombie movie Calvin and Ruby watch (Braindead aka Dead Alive) is shown out of sequence, first with scenes from the climax, and then some from the beginning.