Version:           3.5.6 (using KDE KDE 3.5.6)
Installed from:    Ubuntu Packages
OS:                Linux

This bug is repeatable (on my setup at least). Here is how to recreate:

Here is the exact code in the css file at the start, including white space:

#links a {
    float:left;
    background:url(../img/tab-left.gif) no-repeat left top;
    margin:0;
    padding:0 0 0 9px;
    text-decoration:none;
  }

#links a span {
float:left;
display:block;
background:url(../img/tab-right.gif) no-repeat right top;
padding:3px 14px 3px 5px;
font-weight:bold;
color:#3B3B3B;
}

/* Commented Backslash Hack hides rule from IE5-Mac \*/
#links a span {
    float:none;
  }
/* End IE5-Mac hack */



Then I go into CSS Selector Dialogue. Add #test. Ok. Back at the file code. Here is what is left in my file:

#links a {
    float:left;
    background:url(../img/tab-left.gif) no-repeat left top;
    margin:0;
    padding:0 0 0 9px;
    text-decoration:none;
  }

/* Commented Backslash Hack hides rule from IE5-Mac \*/

#links a span {
    float:none;
  }

/* End IE5-Mac hack */

Now there is only one "#links a span" entry. I guess the bug maybe to do with repeating the same selector in a file? It is needed here to overwrite a setting for the ie5-mac hack.