I think it happened to me when I replied to an issue and then previewed it. But i'm sure I've done the same thing at other times, so it must have been another factor as well. The only way I could then find and view the ticket was to go in to "Recent Posts". Derek Wright wrote:

this is really evil and scary. there are 56 project issues on drupal.org where the pid (project id, the nid of the project's home page) doesn't match any nid on the system. :(

http://drupal.org/node/56454

in particular, commment #6.

anyone have any ideas? please followup to that issue.

thanks, -derek (dww)