Last modified: 2008-01-27 02:04:38 UTC
A fullurl used inside the #tag parser function (see URL) causes &action=edit to become &action=edit, which has been banned on WMF wikis. (Thus, the product classification; I don't think this would cause an issue elsewhere.)
It has nothing to do with #tag or the new preprocessor. <imagemap> has been broken for external links with ampersands since the external link feature was introduced last April.
Fixed in r30178.