OK, I'm not trying to be a wise-ass, but.....Everybody is saying it is VERY dangerous, and a VERY bad idea.....but not specifically WHY. One post said "if just one of your blocks is not glues tight enough, it's rip free and up into the cutter head, possibly crating shrapnel and wrecking the machine." OK, that's the closest. NOw, tell me how the whole board is going to come flying out of the machine.
The board is being fed against the knives, not with them. Aren't all planers set up this way? The rollers are pushing the wood foreward, and the knives are coming down and cutting toward you. How is the board going to go anywhere? In general, standing in line with this type of operation is not a good idea in the 1st place. I run my planer mostly from the side. Also, on my planer, if there were any shrapnel issues, I doubt that any of the pieces would escape the machine at a speed great enough to cause any real damage. Unless you are bending over sticking your head down there watching it go thru. Also fundamentally not a good thing to do. On planers, the business part is pretty enclosed.
On a jointer, yeah, if something goes wrong, there is much less between you and the knives. I worry a lot more about this one.