It's shocking to think that someone would carelessly misplace a vial of an airborne infectious agent with a mortality rate above 20%
We don't know for a fact that this particular copy of Smalllpox was one of the highly fatal versions.
I'm sure this was not careless, as it was appropriately stored. They apparently just lost track of the fact that it was there and where it was, in terms of recordkeeping and careful management of the research specimens.
Seeing as the vial was quite carefully freeze-dried, sealed, and placed into the cold storage, in
a lab where dangerous specimens would ordinarily be stored, requiring the appropriate training of staff for safe handling of such samples: it was really no danger.
Cold storage in vials boxed up is not unusually risky treatment for an infectious agent. I am sure if you looked at more dusty boxes in the cold storage at the various laboratories and regulators, you would find numerous examples of very serious highly-infectious agents, including plenty of examples of Ebola, Marburg, Nipah, SARS, West Nile, Poliomyelitis/Polio, Hepatitis, Pappataci (Yellow Fever), Measles, Spanish flu, HIV, Tuberculoisis, .
A common infection that killed more people in the 20th century than all wars put together.
Smallpox didn't start in the 20th century; its prominence in the 20th century was a culmination of over 500 years of infecting humans.. in the early 20th century, there were many diseases, and it's not so clear to what degree Smallpox actually cut lives significantly shorter than they otherwise would have been. Smallpox caused a lot of deaths, and there were highly virulent strains that developed, but most strains were not so highly deadly and not necessarily airborne either; Variola Minor vs Variola Major, etc, etc.... It didn't kill all the humans(TM) like the black death almost did, else, we wouldn't be around to talk about it, as Smallpox was very tenacious and nasty.... but not necessarily the absolute worst virological threat that we have known as a species.