Inadvertent mixing of chemicals

On May 23, 1998, Employee #1 mixed a cobalt solution and Methyl Ethyl Ketone Peroxide (MEKP) together when the solution exploded and fatally burned Employee #1. Employee #1 was believed to be in the process of pouring excess liquids, which accumulated in the top measuring cup, back into the original container. Employee #1 inadvertently poured the cobalt solution into the MEKP bottle. These chemicals are violently incompatible. The label on the cobalt solution was illegible.
Source: osha.gov