In the OpenCV documentation for fisheye::calibrate it says that turning on the flag fisheye::CALIB_CHECK_COND means that "the functions will check validity of condition number", but I have no idea what that means.
Here is the source code for fisheye.cpp.
What does CALIB_CHECK_COND flag exactly do?
This flag is also relevant for this question, but no one seemed to have figured it out.