When Sam and Dean arrive at the Pike's house to warn them, Sam says that it is 12 a.m.. Right after they get attacked by bees and flee to the house's attic. The bees get in and attack everyone. And then they see the sun rise and the curse is over.
In the story only a couple minutes pass between entering the house and the end of the curse. And no indication is made to more time passing. But around the spring equinox the sun rises about 7:30 a.m. in Oklahoma.
Also the next day no one has any bee-stings on their faces, necks or hands (the parts that would have been exposed to the bees).
After Sam and Dean take shelter in the Pike household, the bugs chew through the phone and power lines, turning the lights in the house off. When Dean enters the kitchen to search for bug spray, the lights in the kitchen are still on.
when Sam and Dean are looking up through the hole in the roof (39.45). the roof is is made of wood roofing shingles, which would not have been used on a modern housing development at that time. also those wood shingles have seen a lot of weather damage.
This episode is set in 2006, but March 20 did not fall on a Friday that year. It was a Monday.
When Sam and Dean are investigating the real estate agent's house, Dean picks up a towel that was covering spiders. They are clearly small plastic spiders commonly used for Halloween decorations.
Near the beginning, when Travis runs over to the hole with the rope to save Dustin, as the camera pans over with him you can see the shadow of the camera and its operator on the ground by the hole.
After Dean and Sam speak with the tribal man, as they get into their car you can see crew shadows and a boom mic in the boot of their car.
When the saleswoman is being menaced by spiders, a black string can clearly be seen leading it across her forehead.
When Sam asks the anthropology professor if there were any Indian reservations in the part of Oklahoma in the 1830s, the professor does not reply that Oklahoma was, in effect, one large Indian reservation (to which tribes from what are now the south-eastern United States were forcibly relocated) until opened for non-Native settlement in 1889.
When speaking to the tribal man Joe White Tree, he misuses the word "Calvary" instead of "cavalry" when talking about the past events of his people.
After the woman is killed by the spiders, we see a police car drive off with its flashing lights on, but they turn off shortly afterwards; and the headlights remain off, despite the fact that the officers are using their wipers and it is raining.