You get the tech wizz that has spent his 2 years of experience studying java memory model and think with your 20 years of experience you should know everything he knows. So you get to talk for 2 hours about JDK implementation details, optimisation of algortithms, the mandatory compiler questions or other he remember from uni. Because you worked last year on a big migration to the cloud that included as a minor item a JDK 11 migration, you need to be aware of all the garbage collector options and algorithms that he so wish to use when they eventually migrate the stuck in Java 6 app the position is for.

So the guy is disapointed, and sure that's not what you are supposed to be hired for, but that's a black mark on your set of interviews and if they don't need to hire immediately you get passed on and understand why the job offer has been opened for 6 months.