The opening scene shows the crash that caused Audrey Rose's death as a head-on collision, but later her father says that the car she was in simply "ran off the road."
The school for girls where Ivy was sent during the trial was administrated by a character dressed as a Catholic nun and addressed as "mother superior". In the mid 1970s Catholic schools still had not fully embraced the celebration of Halloween due to its secular roots. Therefore, it is highly unlikely a Catholic school would allow a ritual with such pagan undertones as students dancing around a large bonfire to melt a giant snowman while chanting blessings for an early spring.
When Janice Tempelton first suspects that Elliot Hoover is stalking her family she enters her apartment 901 (9th floor) she looks out the window down to the street, but the angle that is shown couldn't be higher than the 3rd or 4th floor.
When Ivy is undergoing regression hypnosis each moment would come back in reverse of how it actually happened when Ivy/Audrey Rose experienced it. Therefore the very first moment that Audrey Rose would be in during Ivy's regression would be the fire that killed her -- not, as the movie shows, Audrey Rose enjoying the drive with her mother.
In the first nightmare scene with Hoover present, you can plainly see that one of Ivy's fish is dead.
When Ivy starts walking and crawling into the bonfire in the grounds of the Catholic school, she has several other girls dancing and singing around the bonfire with her, and despite the Mother Superior screaming and yelling "Stop her!" from the window, all of the girls dancing and singing around the flames right next to Ivy seem oblivious to what she is doing and continue with their merriment, and only react to her actions when the nun finally notices the screaming and pulls her back to safety. Only then do they stop and start screaming, despite the fact this has been happening right in front of them for a couple of minutes.