I take a weekly Barn Hunt class that's two handlers and half an hour long. There was a new dog in the class ahead of us and the teacher used two rats in a wire cage to introduce the dog to the rats. After that session was over, the teacher and her assistant were joking maybe AJ needs to be introduced to rats, so they put them on the floor in front of him. AJ went nuts, frantically pawing the cage and singing his song. The rats were put away and AJ went onto the course to find four rats. He went bananas and found all four rats in under a minute! I think from this experience that it doesn't hurt to let the dogs see the rats once in a while.
When I judged Master in the event above, I had a strange situation with the tunnel. The first dog of the trial entered the tunnel entrance on the right twice, stayed in a while, then backed out. After two tries, the handler took the dog to the other entrance and and the dog tunneled with no problem. The second dog in the blind had the same issue. When two dogs do something strange, it's time to investigate and I crawled into the tunnel to see what was wrong. There was a dead end in the build.
This is the tunnel as I designed it, which is legal and safe.
As Designed
This is the tunnel as I inspected the base layer. Note the red square which is a support pillar that I wasn't told about when I designed the course. The bale was tilted to account for the pillar, but the tunnel was still eighteen inches wide. I inspected the base layer, then left the ring to sit down for a few minutes and answer questions from handlers. I returned when the completed three level course was finished and the course appeared to be as I designed it.
As Inspected
When I crawled into the tunnel, this is what I found. The tilted bale was moved in such a way a blind alley was created, which is very illegal. The dog ran into the tunnel and when it encountered the wall, was confused, so backed out. After I left the ring, the course builders changed the bale so they could put the tunnel boards on top of the tunnel. (Note that the bales aren't aligned as they were in my design. That often happens because bale sizes vary and tweaks to the course have to be made during the build.)
As of Final Build
If your dog is acting strange in the ring, you can ask the judge to verify an element, a rat tube or anything you believe is causing an issue. If the cause is the fault of the build or the staff, you can ask for a rerun. Even if you still Q, if you think you can get better time, you can ask for a rerun.
In my first judges workshop, we were given a story of the havoc caused by blind alleys during the first years of Barn Hunt. An Earth Dog judge designed a 40 foot tunnel with blind alleys and multiple turns. Dogs that weren't earth dogs got stuck in the blind alleys and after that experience, refused to tunnel any more. The regulations for tunnels were changed to prohibit blind alleys.