javascript in text widget not working

This is a topic which has come up before, but none of the previous posts match our situation, hopefully someone out there has the answer!

We have a multisite setup with about 50 blogs and 500+ users. It’s deliberately pretty basic with almost no plugins so when a blog admin wants something special, they often end up putting JavaScript in a text widget. Works fine – well, it works fine for about 499 users. I have one exception:

• User A (a blog admin) pastes Javascript into text widget
• User A clicks ‘save’ and the script tags vanish… d’oh!

That’s not unusual. I’ve seen similar posts that describe that symptom, but how about this, following on from step two above:

• User A logs out and I log in… in the same browser
• I paste the same code from the clipboard and click ‘save’ = no problem

I have searched for any reason why one admin user account would behave differently to another and can find none. Dozens of other blog admins do this without a problem. It doesn’t seem to be related to any options or settings or permissions – everything else the admin needs to do works as expected. This morning’s test proves it’s not related to the browser or computer.

Actually, I lied about the numbers – this is the second user I’ve seen with the problem, but the last one was several years back when the blogs were still on a 1.5.x version, so I didn’t think it was relevant.

Any assistance gratefully accepted!