I remember a story a professor once told me about something similar to this. He once knew of a research team that was trying to develop an algorithm to tell the difference between pictures of American and Russian tanks. They were able to achieve a very high success rate very quickly. Excited but skeptical, they decided to keep testing the algorithm on lower and lower resolution photos. Shockingly, they were still getting close to 100% identification on images of sizes around 10 by 10.

Turns out, all the pictures of Russian tanks were taken in the winter, while all the american ones were taken in the summer. All they had done was trained a model to classify how bright the picture was.