Ringer's Solution (It can pay to be sloppy)

125 years ago, a physiologist named Sidney Ringer discovered that a solution of saline prepared by his assistant seemed to keep excised rat hearts beating longer than normal saline.

It turns out this is because the assistant was a lazy slob and used London tap water instead of distilled. Good thing, too - the water, being hard, had a substantial amount of Ca²⁺, which we now appreciate is important in muscle function.

A derivative of the solution (invented only 50 years later) still lives on, Ringer's Lactate.