When Lyudmila and her friend approach the office building she is wearing a pair of white shoes. Once inside the building the shoes color changes to black.
When Katerina is talking to Gosha in the elektrichka train the position of the hand bag besides her changes between the shots.
When Katerina meets her lover, they discuss how they haven't seen each other for a long time, and that their last meeting was on June 18th. But when she gets home the same night, the calendar shows that the month is still June (a large "6" is visible).
During the farewell scene of Gosha and Kolya you can see the microphone shadow on the wall.