The Greater Good (a.k.a. Sides) - S1-E21
Continuity mistake: Just as Sayid walks out of the building (number on both doors is 815), the white van pulls up. When Sayid gets in, in the exterior shot, the door he opens is opened to Sayid's left, but in the next interior shot, the door is opened to Sayid's right (the door to his left is closed). In this shot the building is seen behind Sayid - note the number 815 on both doors have vanished, and the chipped wall paint differs. (00:33:55)