I've only experienced this in IE (I have version 7.0.6001.17128). In the following example, after the sample div has faded out and just before it fades in, it flickers briefly.

<script>

function toggle(e) { e.is(':visible') ? e.slideUp(1000) : e.slideDown(1000) }

</script>

<input value='slide toggle' type='button' onclick='toggle($("#jtest2"))'>

<div id='jtest2'>

here is a jquery test<br>

here is a jquery test<br>

here is a jquery test<br>

here is a jquery test<br>

here is a jquery test<br>

here is a jquery test<br>

here is a jquery test<br>

here is a jquery test<br>

</div>