O Brother, Where Art Thou? - S4-E15
Character mistake: During the opening scene, while Lex is talking to Lena he says he was quoting Epicurus from 230 years prior. Epicurus died in 270BC, making that quote around 2300 years old.
O Brother, Where Art Thou? - S4-E15
Factual error: While the location of National City is never revealed, it is generally accepted that it is located in the US. But all the helicopters used to fly Lex around are Canadian.
Answer: For one, it's usually cheaper to film in Vancouver because of the tax breaks. But all the CW Arrowverse shows (with the exception of "Black Lighting") are/were filmed in Vancouver. So it makes it easier and cheaper for sharing set designs, crossovers, etc ("Black Lightning" filmed in Vancouver during its crossover). Originally "Supergirl" wasn't part of the Arrowverse when it was on CBS, but it was too expensive to produce and wasn't reaching the wider audience the CW shows had. So CBS moved it to its sister company, The CW.
Bishop73