When the Chinese restaurant window breaks in 1929, it is clearly tempered glass. Although tempered glass had been invented, it was not in widespread production or use in 1929, and no restaurant window would be made of it.
The attempt to prevent World War 2 by killing Hitler was made in 1940, a year after the war began.
Sometimes no one notices time has been altered and act as if it's always been like that, other times they know as it's happening.