Why am I getting garbage value instead of RGB while printing out colour of pixels?

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This snippet of code prints out garbage values.

cv::Mat_<cv::Vec3b>::iterator it = img.begin<cv::Vec3b>();
cv::Mat_<cv::Vec3b>::iterator itEnd = img.end<cv::Vec3b>();

for(; it != itEnd; ++it)
   std::cout << (*it)[1] << std::endl;

But this prints out the vector of RGB values.

std::cout << img.at<cv::Vec3b>(100,200);

The moment I try to access the individual RGB values I get garbage values printed,

Any reason why this might be happening?