A Borg drone is created by a transporter accident.
A transporter accident merges the Doctor's mobile emitter, which comes from the twenty-ninth century, with Seven of Nine's Borg nanoprobes. The emitter starts assimilating a lab and builds a maturation chamber that grows a new Borg drone. Because the drone is made from future technology, the Borg could gain a dangerous advantage if they find it, but Janeway lets it mature in the hope of teaching it to be an individual.
The drone, who takes the name One, learns quickly with Seven as his mentor, and he grows curious about humanity. His signal draws a Borg sphere, and One helps the crew fight it, then beams over and steers it into a nebula where it is destroyed. Badly hurt, he refuses treatment, knowing the Borg would hunt Voyager endlessly to obtain him, and dies. The fifth-season episode gives Seven a role like a parent, and her grief over One shows how far she has come.