Just think about the mass of all microscopic organisms outside the labs that suffer mutations all the time. Worrying about innocuous bacteria turning into something bad on accident within comparatively very limited environment of a laboratory doesn't make much sense. Think also of the many free floating bacteria cells in our environment that die due to various factors, whose bodies then have to break apart. Compared to what may happen in a lab, I imagine that those pieces of dead cells floating around freely may pose a greater risk of falling and fitting somehow within living host cells altering their function. (That's one of the mutation vectors, BTW, be it constructive or destructive.) The only scenario that makes sense to seriously worry about when a lab works with/on a virus or bacteria is if the material they started with carries inherently a high risk, or (worse) if they focus on weaponizing it (but that would not be "unknowingly" as you mention).