Editor settings | Strip trailing spaces option: set "Modified lines" value. Prepare a file with trailing space like:

public class A {
    private int untouchable; // 1 space after: 
}

Modify the line with untouchable. Then UNDO. File is indicated as not changed (it can be easily observed if you use "Mark modified tabs with asterisk" option). Go to another line, e.g. to the very first one in file. Modify it (press Enter). Save. Result: trailing space is erased in "untouchable" line. This is not correct. Changes in that line were undone.