This is the only photo that looks like this, others taken before or after look fine, so I don't think it's dust or oil in the camera or on the sensor. Photo was taken during summer months, so could it be a bug that flew in the frame at just the right time? Leaning towards no, since it appears to be roughly square in shape but oriented like a diamond.
Camera is Canon DS126071 (Canon EOS Rebel XT 8MP DSLR)
ISO - 1600
Shot - 1/640 sec. f/16 22mm
Edit - let me be absolutely clear, I mean the giant black square over the subject's face, that wasn't intentionally put there to mask his identity before posting.