Factual error: One of the deaths involves a burning candle, a car and a leaking fuel tank, resulting in an explosion. This wouldn't happen. Multiple independent test evidence that petrol/gasoline can't be ignited this way, a flame isn't hot enough. Only the fumes can be ignited, which isn't what happens here, and the tank wouldn't explode anyway.
Happy Death Day (2017)
1 factual error
Directed by: Christopher Landon
Starring: Israel Broussard, Jessica Rothe, Ruby Modine, Charles Aitken, Rachel Matthews
Continuity mistake: When Tree looks at her iPhone it shows the time as 9:23 when the power goes out. Then when the TV is shown the time on that clock is 9:24, when she walks to the TV the time is then 9:31, and then the next view of the clock shows the time back at 9:24.
Lori Spengler: How did you figure it out?
Tree Gelbman: Because you've killed me before.
Lori Spengler: Then I guess I'm just going to have to do it again.
Trivia: There actually is an in-universe explanation for why Tree got stuck in a time-loop. However, director Christopher Landon opted to omit it from the finished film. Partially because it could be used in a potential sequel, and partially because it wasn't necessary for the narrative. Landon has hinted that the upcoming sequel, "Happy Death Day 2U," will finally reveal the explanation.