g_axelsson said:
One way contamination can travel from one beaker to another is when you have HCl and nitric in beakers beside each other.
When I was in high school, I worked as a chemistry assistant in the back room mixing all the solutions other students would use in class. We learned to work very accurately and cleanly. We referred to nitric acid as a "creeping" acid. Even when poured cleanly into a beaker, with no splashes or drips, if we picked the beaker up later we could end up with yellow fingers. I didn't understand the mechanism then, so we just thought of the acid as being able to creep up and over the edge of the beaker.
Dave