Continuity mistake: At the very beginning of the film we see the Seminar Leader start to write eXistenZ on the board. We see him write the letter 'e', but then the camera changes and he starts to write the letter 'X'. Notice that the letter 'e' has changed from how he originally wrote it. Then when he finishes writing eXistanZ the camera switches position again and the writing has all changed the look again. Most noticeable by the crossing of the letter 't'. As we saw him write it, then cross portion was much higher than in the wider shot.
Continuity mistake: When Ted and Alegra are escaping from the seminar we see them in the car driving through the countryside. Ted's phone rings and Alegra grabs it, throws it out the window which she then shuts. A few moments later, Alegra informs Ted that anyone can use his phone to track where they are. The camera then changes angle to face Alegra, and the window is now open again.