An extra is in two places at the same time. In the opening party scene, you see a blonde woman in a bright patchwork sun dress chatting inside the house. The camera travels through the house to the front yard; the same woman can be seen chatting behind the two lead actors.
When first shown in the house the A/C is clearly off and closed.
However, further scenes show it on and the fact it is on is used as a clue in the investigation.