I shot an image of a street reflected in a large fisheye mirror. The telephone lines in the background and everything else was fine. I gave it a very slight crop (less than ten percent) and suddenly the telephone lines became dotted lines like morse code. I did preserve aspect ratio while cropping. What can I do to fix this and why did it happen?
A. There is a good chance that the issue is only in your display software (meaning not in the picture). If the number of pixels in the display is different of the number of pixels in the picture, the software needs to make it fit somehow. Depending on the level of sophistication in the code, it simple skips every Nth pixel row, which results in potentially the only pixel row with the telephone line being not displayed.
B. (Less probable - the result would look different) If you save your picture as JPG, you need to understand that JPG is not generally lossless; most save dialogs give you the option to lose between 0% and 99%. The default is mostly 20% loss, but it depends of course on your software and settings. So if you edit anything and then save, you lose 20%, next open and save another 20%, etc.