Good question Ross. I will restrict my answer to a professional mistake!
After I graduated from college and became a biologist I never had any opportunities to practice my math skills. Over the last few years analysis of enormous, complicated data sets has become more important in biology and it took me some time to practice my math and get back in shape.
What a big question! I will also restrict myself to a professional mistake…
Part of my job involves ordering new parts for our machines that make up the production line. New parts need to be ordered as part of maintenance tasks because some of the machinery wears out over time- you cna imagine why it would do this when you think these machines are very fast and work 24/7!
I once ordered all the wrong parts for one piece of machinery and had to spend A LOT more money that I was supposed to to put it right!