When Nathan Petrelli is standing in the doorway to the bathroom, through the doorway you can see the moulding at the bottom of the counter, and the wall past it. When he steps into the bathroom, it's clear that there are the legs and feet of a body in front of that same moulding. When the camera is distant again, the feet are gone once more.
When signing in Claire has a clue that Nathan is being impersonated my Sylar because he writes with his left hand, however throughout the entire show when Sylar cuts open someones head, cuts his own arm or even uses a screwdriver to fix a clock he uses his left hand.
At the very end, when trailing season 5, someone is seen reading a paper with an article heading of "Fourth Mysterious Drowning ...". The text of this article is just several repeated paragraphs and bears no relation to the heading. The same text also appears in adjacent articles.