Generally speaking a jig bore is intended for making precision holes. The head will feed. Most vertical mills vertical feed by the table moving up and down. The "quill" of a jib bore is more robust and a precision fit piece.
Speaking of grinder boo-boos, had a couple when I was an apprentice. Fortunately, they were not my boo-boos. One was on a Blanchard grinder. The big mag chuck was loaded with 1 inch square steel blocks. There had to be at least 500 of them on the table. The apprentice had just finished setting them all up and went on break. The instructor, who was an anal old guy, walked by, saw the machine sitting idle and noticed that the mag chuck was still turned on and he shut it off. You can guess what happened. The apprentice had already touched off on the pieces before he went on break. So, when he came back he started the machine and hit the auto-feed. There was this crash/explosion of noise in grinders and you saw guys diving for cover everywhere. Those little 1 inch blocks punched holes in the l walls and we were still finding them months later in various nooks and crannies in the shop. The instructor tried to blame the apprentice. But he pushed back and the instructor finally admitted he'd turned off the mag chuck.
The other was on a cylindrical grinder. They have a hydraulic rapid travel in and out. So you set up the piece on centers, rapid travel in. Then with the hand wheel slowly feed to touch off and then start to grind. We made tensile specimens for the materials lab in the plant. So, to check the diameter, you rapid feed out, pull the piece off and measure it. Then rapid travel back in and take off more if required. So, the apprentice backs the piece out, measures and runs it back in. But the machine must have had dirt in the hydraulics and it does not go all the way back in. The apprentice gets distracted, probably by someone talking to him. He goes back to work and forgets what the measurement was. The piece is not against the wheel so he assumes he did not send it forward. Just as he reaches in to grab the piece, the obstruction in the hydraulics clears and take the piece into the wheel, along with his finger. It was ugly.