I realize that deer ribs are virtually a non-issue on most shots. It's certainly nothing I've ever worried about. Just wondered though as strange events happen. I'm searching for an explanation for something I just experienced. Here is my sad tale. The other day I shot a nice buck from 20 yards on the ground. (552 grain Beman MFX carbon with Grizzly 2-blade out of a 57# Blacktail.) I have consistently had full pass throughs with this bow. Arrow placement looked very good; 1/3-1/2 way up the body and a few inches behind the shoulder. When he ran off, I was expecting to see him fall except I noticed most of the arrow hanging out and angled down but not flopping. Height matchd the height I thought I saw at impact. I watched him run full bore for about 150 yards. No blood, no arrow, no deer. He never slowed down. Not a single drop! Lots of fencerows and open fields beyond so I expanded my search there the next morning and afternoon. I'm sure I was nowhere near the shoulder. I think my arrow must have deflected off a corn leaf or something. Replaying the events in my mind, I don't know how I didn't get this deer. Maybe I deflected both off the corn leaf and off a rib. Anyway, nothing makes sense as I've never experienced anything like this before. This arrow flies extremely well out of my bow by the way. I know that it's possible I didn't hit where I thought, but I don't think I'm wrong on this as I watched him run so far with my arrow and saw impact. And I'm not suggesting this deer's ribs stopped my arrow either unless it was already deflected. At this point, I can only speculate what went wrong.
As for the elk rib...it didn't happen to me, but I know the story. Someone I know shot an elk with a 54# Thunderhorn bow, Beman MFX carbon arrow and 125 grain G-5 broadhead. Overall arrow weight was obviously pretty light. Arrow the shot, the elk ran and stopped, and he got a real good look at placement and (lack of) penetration. He concluded that he smacked the rib dead center as the arrow was perfectly positiond behind the shoulder. No blood, no penetration, no elk. Since then he has upgraded to about 650 grains and Grizzly 2-blade. The shot he had on an elk this year turned out much better!