Crushbone pathing with pet falling through ground
 

We're not exactly sure what happens, but it appears that pets can fall through the ground in various places in Crushbone. When this happens, it aggros a lot of the enemies in the Castle and then your party gets trained.

This has happened with an enchanter pet while camping the wall next to the castle. Also has happened with a necro pet while fighting near the Slaver cabin.

All the sudden the caster notices his pet is gone and several seconds later 10-15 mobs from the castle come running at you.

I'll confirm.

I'll add that we took a direct route to the slaver caves, entering crushbone, turning left, crossing over the bridge, over the crumbled wall. The necro didn't swimming or anything odd like that. I was told the train went over Trainer Hill, and we weren't, at any time, near Trainer Hill. (Oh what a train it was!)

I'm pretty sure this isn't possible to fix.

If I'm not mistaken, crushbone isn't the only zone this happens in. I believe I've heard of it happening in other zones as well.

I'm also fairly sure that this was how it was in classic, too.

My honest suggestion is to either be very, very careful with pets in dungeons, or to not use them at all. I know, it sucks, but it may not be fixable so may as well learn to live with it.