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tl;dr: hold the board very still or get a camera with global shutter.

Rolling shutter, a property of most webcams and some industrial cameras, can distort objects that are not perfectly still. If you've moved the board even just a little, it will affect everything you do with the picture, starting with intrinsic calibration.

To give you a sense of scale for the distortions: assuming a 30 fps video stream, the worst case rolling shutter lag is 33 ms. A pedestrian travels 40-50 mm in that time. if your hands are moving slightly, you can maybe expect a tenth of that, which is still A LOT in proportion to the checker square sizes most people use.

if you've printed your checker board pattern, make sure to measure the width and height of your squares. they might not be square at all.