When you are filling in a ticket If you forget the category it brings up a box with:

A necessary field "Category" was empty. Please recheck your inputs.

Please use the "Back" button in your web browser to return to the previous page. There you can correct whatever problems were identified in this error or select another action. You can also click an option from the menu bar to go directly to a new section.

If you click on the back button on your browser then it returns you to the submission page but all the fields are blank.

This doesn't seem to affect the 2.x branch but due to the removal of some features we use to wrap MantisBT within our site we can't upgrade at the moment.