When Poldek is taking a bath, a scar from a smallpox vaccine is clearly visible on his left arm. Most of the world's populace wasn't vaccinated before the WHO Smallpox Vaccination program began in 1967.
The goof item below may give away important plot points.
The English subtitles during the end credits state that "On May 12, 1945, Leopold Socha was killed, saving his daughter from an out-of-control Russian army truck." However, according to his gravestone and the Polish text on screen, he died the following year, on May 12 1946.