The first slip-up is when 'The Caller'(Kiefer Sutherland)is talking to Stu(Colin Ferrell)about The Others,as in his other victims.

Stu says,"they were executioners",and he leans to his right and puts his head on the glass of the phonebooth.Then the shot changes and the next second he is standing straight up.

The second slip-up is after The Caller shoots Stu in the tip of his ear.After his blood has reached down to the end of his neck,probably about a minute in a new shot the streak of blood is shorter,then it goes back to normal in another shot.