Similarly, in 1984, a truck carrying unexpectedly radioactive steel rebar took a wrong turn into the entrance of Los Alamos National Laboratory and set off radiation detectors at the gate intended to detect radioactive contamination on workers leaving the site.

This triggered an investigation which traced the contamination to the improper disposal of the active element of a retired medical radiotherapy machine that used cobalt-60 - the radioactive cobalt ended up mixed with a large batch of other scrap metal, contaminating (among other things) ~6,600 tons of rebar, much of which had already been shipped at the time this was discovered...

https://en.wikipedia.org/wiki/Ciudad_Ju%C3%A1rez_cobalt-60_c...