in my installation I configured a few projects with different categories and versions and so on

if I use the "switch" project to go to project1
and then click the link in a bug notification (this bug is in project2) to view the bug and then click "update"
then the category and version and so on list boxes in the update page are populated with values from project1 (which is the current project) instead of the values of project2 (which the bug actually belongs to

now, if you don't notice that because you don't intend to change anything there anyway and save the update, the fields will be cleared