This question is really for Sam Hemelryk, but it seemed like a good idea to ask it here.
On the MUC admin page .../cache/admin.php there is a Purge link next to 'Default file store for application caches' in the Configured store instances section, which lets you purge all the stuff that is cached on disc.
However, there is not a Purge link next to 'Question definitions' in the Known cache definitions section.
Why not? It would be useful to be able to purge things more selectively.