there is an issue and I think you have to solve it.
Its about Mootools 1.2.4 which Joomla 1.5 is using.
A lot of users have this bug, too, so I'will quote them:
SCRIPT5022: Exception thrown and not caught mootools-core.js
I posted a bug report to Microsoft about this and they got back to me.
They say that IE9 doesn't work with Mootools by design because Mootools uses a browser sniffing technique that isn't standards compliant. The IE9 team will not change this behaviour before the final release and it is, in their view, the responsibility of the Mootools developers to fix the library.
I’m not sure what causes it but I forgot to mention that it only occurs on IE9 - there is something about ‘uncaught exception’ somewhere in “newElement: function(tag, props)” (sorry I can’t check exact place right now).
The Mootools Developer says that they have solved the problem by upgrading Mootools to 1.1.5.
But Joomla uses the old one.
Now, what can we do now or what are you going to do?