Page.RegisterStartupScript

I'm using this function RegisterStartupScript(alert("...")) to check to make sure the user chose a selection from a listbox. This works absolutely perfectly on my computer and all the computers in the office, but one off-site user clicks the submit button and the page will not submit. Any idea what could cause this?

I will try my best to help you out. Can you provide more code and show exactly how you are using this?

Does he have an old browser? Javascript disabled?