One of my long-ago Weird Jobs was going through US Army aptitude tests and fixing errors the test-takers had made in filling out the machine-readable bubbles under their names and SSNs.
My first office job back in 1985 was a bubbler for Montgomery County Public Schools. We filled in the "bubbles" on the scantron sheets for all the first graders in the county for their standardized tests. I was always careful and double checked my work, but I worried about errors creeping in.
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Bubbling