The goof items below may give away important plot points.
During the final fight, Chell shoots a portal to the Moon, is transported there, and sees the Earth just over the Moon's horizon. But the portal was shot near the middle of the Moon, which means the Earth would have been high above the ground plane, not on the horizon.
A sequence during the boss fight establishes that the fight was taking place at night as indicated by the dark sky. When Chell escapes, the sun is shining. However, in between Chell being pulled back in from the moon and escaping she blacks out for an unknown amount of time, which could account for the sudden change of time outside.