1) In IE8, go to the full-page Create Issue dialog.
2) Pick a project and pick an issue type.
3) Change to another project that also has that issue type.

The issue type is no longer available for selection.

See video using QA data:
http://screencast.com/t/HgQdolFWAC
(What all the mouse-pointer-waving is trying to indicate is that Bulk Move 1 has the Improvement issue type, but if you select Improvement in the QA project first, then switch to Bulk Move 1, it reverts to Bug and Improvement is no longer in the list).