I think I found the problem! According to the debugger, the client is using IE6.

I hate IE6 just as much as the next guy, with an absolute passion. However the startup that I work for has a focus on teachers, and unfortunately quite a significant number of schools and teachers are stuck in the dark ages with IE6. 23% of our users are still using IE6, so I have to support it. I found this helpful in debugging our soon to launch redesign.

alert("hi");