The Hollywood sign is no longer lit at night.
Monica says "Half the people in this county are living in FEMA trailers". Louisiana has parishes, not counties.
When Derek stops the car to avoid hitting Sylar we see the front of the vehicle has no front license plate. As a California registered vehicle it should have plates on the front and rear bumper.
Adrian Pasdar is missing half his thumb tip. When Matt gave photos to Nathan in the hospital, you could clearly see that Adrian had a "hands double" when he was looking at those photos.
When Matt is explaining his ability to Nathan, he begins talking with his back to the camera. However, his jaw isn't moving at all, indicating that the audio was added later.
In the episodes where Peter Petrelli is in Cork, Ireland, none of the actors have authentic Irish accents. Their faux accents are more like Dublin or Northern Ireland accents, rather than the distinctive Cork accent (Cork is on the South coast). What is more, Caitlin has a faux Northern accent while her brother has a faux Dublin accent.