This happend to me when I tried to load ajax-content in internet explorer (worked fine in Firefox, etc), which had a script-tag without a valid comments tag.

So just for the record, here is the solution, which helped me.

Before (Broken):

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<script type="text/javascript">
<!--
alert('text');
// -->
</script>

Fixed (Working)

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<script type="text/javascript">
// <!--
alert('text');
// -->
</script>