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May be I'm wrong but I can reproduce problem :

import cv2
from matplotlib import pyplot as plt
test = cv2.imread("F:/Images/20150618_185237.jpg") #image size is 4128X3196
test[:,:,0]=test[:,:,2]
test[:,:,1]=test[:,:,2]
t = cv2.rectangle(test,(871,700),(1071,560),(0,255,0), 2)
plt.imshow(test)
plt.show()

Now results are : no rectangle, a weird rectangle and full rectangle.

Sometimes problem origin could be multiple...