The Biggest issue is really looking for a good employee that only wants part time low pay employment< by good I'm not meaning trained machinist I'm meaning shows up, can follow basic directions, can repeat those directions.
you reference your cleaning lady that works cheep and does a good job, she likely does the same work for multiple people and that makes for full time employment. I think its super unlikely you will find a agency that has someone with mechanical aptitude, because if they are good enough to make you happy 2 days a week they are good enough to make every other employer happy 5 days a week, and that's better for both of them. Now if you tell them to send whoever 2 days a week your likely to be starting from scratch every week or likely day, also if you are counting on the labor to get projects done you will find that the % of no shows for temp jobs is surprisingly high.
easy,
Put part in but didn't tighten it enough, just enough to let the probe work but flies out when the mill starts cutting, or put blank in wrong orientation so its too tall and your probe crashes into the blank that shouldn't be there, or left the torq wrench on the vise and the spindle crashes into it, or 30 other things you can't imagine would be a problem because you wouldn't do them yourself.