My favourite is was when I was playing jumping in an instance on Alderaan.. and jumped out of the instance. I was somehow inside the main map's mountains.
Oh it really grinds my gears when that happens because the only way out is usually to instant travel out. 95% of the time /stuck just drops you somewhere else inside the terrain. Still, at least it's rare enough that it's not a big deal and pretty darn cool in side of there.
There should be like codexes for some of these places that takes you hours to get to even if you know how to, and then like a title when you collect so many.