I experienced many times that when watching a videotape, the picture became white and noisy at points where otherwise it does not. I reversed the tape, watched that point again, and the noise was not there. So, it occurred only once at the given minute(s). The noise can be just a white line or the whole picture is white (or between these two; e.g. half-picture is white).
This happened on both newer and older tapes; both on tapes recorded over many times or recorded only once; on different brands of tapes; so these cannot be the cause of the noise I think. I also experienced it on different VCRs (2, 4 and 6 heads). So it does not depend on the condition of the VCRs and the tapes, either.
At some points, there are also recorded white lines/noisy parts, and these are there whenever I watch the tape. These errors occurred during recording, and they got embedded, so I know they cannot be eliminated. However, those occurring during playback and only once, could they be prevented? But how?