Looking at a file with 'less' in a Linux session, if I scroll down and then back up, 'Esc-M' characters appears at beginning of some lines. I didn't catch exactly the cases, but they are related to lines with a hash ('#') and then some empty line.

For example:

1. Execute 'less /etc/services'.
2. At the beginning of file there are some lines with comments (they have a '#' at the left side).
3. Scroll down with the cursor key so the comment lines disappear.
4. Scroll back. The comment lines appears again and some of them has a 'Esc-M' at the beginning overlaping other characters.

And this is how it should look:

Cheers!