Recently I was recommended to use Mend regularly as a safe and prudent procedure. Foolishly, I used 'Mend and Prettify' rather than plain old vanilla Mend.
While scrolling down through an xhtml file to select all the body content, I was surprised to see so much white space passing before my eyes in Code View. When I investigated, I found Prettify had reworked all the <tables> so every cell in every row occupied four lines: one each for <td>, content, </td>, and a blank line. Plus <tr> on its own row as was </tr> and with another blank line between them.
The tables were not very complicated, and I had built them with 3 to 4 cells per line in CV and I was quite happy to have </tr><tr> on the same line without any empty lines.
Prettify added around 600 extra lines to each of the files which had tables. This doesn't cause any problems for the machines but it makes editing hard for me to see where I am in the table.
It took a little while to clean up but I think the message (for me at any rate) is not to use Prettify if you have some tables in the file.