Dimethylmercury is (or was, at least historically) used as a reference material for nuclear magnetic resonance (a common analytical method used in chemistry):

https://sites.northwestern.edu/ohalloran/199hg-nmr-standards...

This MIT incident was in a chemistry building that has been in use since 1970, so it could have been accidental release of an old, unused container or sample tube of the material.

Thanks! 40+ years ago I worked in a marine biology lab as a dishpig for seawater analysis and we had fun times when the annual clear out found a gallon jug of benzene at the point of panic about benzene being replaced by toluene.

The wording of the announcements, sounds like a student synthesized it. Can't imagine why, hopefully it wasn't a fatal mistake.