Please attach a small 2-question sample survey demonstrating the issue. Thank you.

Here it is.

Think it was not a bug.

The "multicheck question" don't have any answer, if you check one: no .missing class.
When you come back : the question don't have input-error but only missing class.

Maybe the "problem" is the css:

.outerframe.withindex div.missing td.questiontext {
background: none repeat scroll 0 0 red;
}

This one apply for error or not, and red is clearly an error.

Thank you, I think I understand. Will you commit a fix to the CSS then?

Fix committed to master branch: http://bugs.limesurvey.org/plugin.php?page=Source/view&id=11643

Need reviewing for 2.1/Plugin

2.00 Build 130219 released