Here's a fun one for Friday afternoon.

I was trying to think of some nasty XSL questions to send to people
but couldn't come up with any.

What do you consider a nasty XSL question, is there such a thing?

Do you have any suggestions/sites that give some?

Ask simple questions.
I had a similar problem and come up with a simple questionnarie:
http://www.nesterovsky-bros.com/weblog/2008/07/28/OnTheJobApplicants.aspx

Untrivial xslt task: "please give an example of exception handling in xslt".
http://www.nesterovsky-bros.com/weblog/2008/07/31/SFINAEInXsltOrNativeExceptionHandling.aspx