@murrayjames, seamus5,

Yes - the problem is related to the system wanting to test someone on a specific word, but not having the necessary information. If there are multiple testing options available we try to switch to a test that takes advantage of whatever information is there, but we can't guard against all edge cases and still give people control over customizing their test options.

In an attempt to solve this, I've just followed drummerboy's advice and put in a quick interstitial page that will prompt people to provide missing data for these words instead of just throwing up an error page. Hopefully this will make it clearer to people what the problem is while giving them a workaround. Let me know if you guys run into any trouble with it.