In what was a potentially fatal gaffe, some HIV-positive US military personnel were vaccinated against smallpox; had any of the vaccinees been anything more than HIV-positive (i.e., had they had full-blown AIDs), even the weakened smallpox virus present in the vaccine might’ve put a serious crimp in their, um, living.

(On a more positive note, the incident did spawn some constructive debate on what exactly should be policy regarding smallpox vaccinations and HIV-positive people who also happen to be at risk of contracting the basically extinct disease [i.e., we're assuming military personnel here])

(via EurekaAlert: “HIV-positive U.S. military personnel accidentally vaccinated against smallpox” [April 15, 2004])