I saw an example just this past week, on Vandy's "Statue of Liberty" play. That play never should've worked if the defense was coached properly. I've attached a link for you here.
Vandy
First, watch our D-line. They clearly slant to the defensive left at the snap of the ball. In particular, you can see Wayne Dorsey (7) slant inside, which means he is no longer capable or responsible for containment. The DTs and Rivers (90), the other DE, slant as well. They actually get penetration with this slant, as you see on the film.
Now, anyone who has a basic knowledge of defensive principles knows that if you slant, you have to still maintain gap responsibility using your LBs. If you slant your DL to the left, you better have your backers shifting to the right to be in position to fill the gaps left by the slant, in particular having someone to fill in for containment on the back side. In this set up, we bring Temple (4) on a corner blitz. I assume he's supposed to be in contain, but he's so far outside that he isn't able to fill in for containment. That means either he didn't come down to fill the outside gap containment, or the LBs should've been responsible. Given that we brought a corner blitz, that means Kight was probably responsible for filling for the corner, not filling a gap. Watch our LBs on the play. Kight (15) drops back and slides to the outside some, but he's clearly dropping in that zone for pass coverage to fill for Temple who blitzes, not for gap responsibility purposes. Williams (44) actually drops into a coverage zone in the same direction as the slant, leaving basically Kight, Temple, and a safety (20 yards downfield) as the only players on that half of the field period, none of which has a clear containment responsibility.
In effect, Nix opened a wide hole in his own defense for Vanderbilt. All their blockers had to do is get in the way of a couple players, because the hole was already created for them by Nix. That's the ultimate problem Nix has with the read option style offense. He loves to blitz and slant, but he doesn't understand the principle of gap responsibility still has to apply to be able to stop anything. If you move your defenders out of their base positions, you have to rotate players to fill the spots being vacated.
That's why the zone read kills Nix so much. It doesn't get him as bad when he stays in a base, but when he starts slanting and blitzing, all the QB has to do is make the correct read about which way the defense is moving and then either keep or hand off to the side the defense is vacating, and you have a magic hole to run through. You don't even have to block it very well, because he sets himself up to be outmanned significantly. Sometimes it doesn't even have to be a slant for it to work. Since he doesn't teach gap responsibility, his players will just guess at the hand off or keeper and sometimes create the gap for you. You can watch that on Relf's long Egg Bowl run last year. Rather than filling a gap, one of our LBs just chases the tailback. Then both safeties compound it by both chasing the same player, and it turns a 4 or 5 yard play into a 70 yard play.
Egg Bowl