There are so many things one has to deal with when running a lab that never even crossed the radar previously. A list would be sufficiently boring as to drive people away in droves, but simply employing people directly brings up hundreds of possibilities. One I've never seen solved all that well is when someone in the lab is unable to work for a prolonged time due to factors outside their control.
It could be anything, really. Sickness, accident, the health of a dependent, etc. In certain cases (e.g. pregnancy) where there is some predictability it is possible to plan and even bring in additional help if necessary. Of course, our parental leave policies in the US are too restrictive for there to be an easy mechanism in place, but I've seen a short-term staff member do a great job of keeping the science rolling.
The tougher situations are the unpredictable ones that lack a clear timeline for return. Do you hire someone or wait it out? How does hiring short-term help affect the status and insurance of the person they are replacing? If recovery time is faster, are you on the hook for two people for the length of the replacement contract? Do you just let a project hang until the person returns? There's no formula and few good options. This is doubly true if there is a substantial union process for hiring and the injured/sick person is a staff member. How do people handle these situations?
Perhaps sometimes science just takes a backseat to the health of those doing it.