It is easy to spot firearms out of time period so prominent in so many films its not even sport.
50 star US flags in WW2 or earlier films ? Newer cars in vintage period shots or AC units in windows , power lines
....I often catch wrist watches on extras
Usually there is just one continuity person on a crew especially on locations stuff does get by that just cant be foreseen....
Like contrails ( as you mention ) or the pickup truck in John Wayne's " Alamo " and the white car in Braveheart
The wake of the film boat or the tire tracks in chase shots in Westerns.