%30 encodes for 0, so %%30%30 should decode to %00, which is the urlencoding for a null char. My guess is that this either crashes the engine as it tries to open the URL with a null byte in it, or the GUI as it tries to display the tooltip.

A bit of reading on the null byte from a web development perspective: http://projects.webappsec.org/w/page/13246949/Null%20Byte%20...