Pause might give you two fields (or one frame's worth) of video depending on the system. There is no guarantee that the two fields will match in terms of subject position or motion. If the pause gives just one field, the even scan lines put on the screen are duplicates of the odd scan lines.
On a VCR, during pause, the diagonal path the head makes across the tape is not quite the same angle the head makes during normal play. This results in the head spanning more than one recorded track where one recorded track corresponds to one video field. There is also a discontinuity when the head crosses from one track to the next so you may see the picture split into three of four horizontal bands. The better VCR's use four or six heads to scan the tape during pause, and blend things to give a smooth still frame but still, several fields contribute to the picture as seen and blur due to subject motion may be noticeable.