When Cele attends the funeral of the man she has discovered to be her step-father rather than her father, there is snow on the trees in the background; however, one can see that the trees' leaves are green under the snow which shows that they have been intentionally covered with snow and it was probably summer rather than winter during the time of shooting.
During the church scenes with the "God's Trying To Tell You Something" track, a Hammond organ is audible being played through a Leslie rotary speaker in the left channel. However, the Leslie (then known as Vibratone) rotary speaker wasn't introduced until the mid to late forties and it certainly was not marketed alongside a Hammond organ. The Hammond Organ Company and Electro Music, the company which manufactured Leslie speakers, were involved in a bitter dispute over the popularity of the Leslie versus Hammond's own line of stationary tone cabinets.
During the scene where Miss Millie brings Sophia home for Christmas, from a wide shot of the house, you can see smoke coming out of the chimney. When Sophia gets inside, there is no fire in the fireplace.
The postmark on the envelope of the letter from Nettie is dated July 1934, however the letter inside is dated for April 18 (Easter). She opens the letter at Easter (Albert is decorating Easter eggs) so the letter did arrive in a timely manner, however the July postmark doesn't make sense.
When Mister finds Celie out in a field reading one of Nettie's letters and strikes her, the letter, her glasses, and the book she was carrying (to hide the letter while she read) all scatter while falling to the ground. In the next shot, we see a close-up of the book, letter, and her glasses all neatly stacked on top of each other as they land on the ground.