Today we're dealing with a rude joke from Till Eulenspiegel - animal lovers shouldn't read any further...
The 46th story from the Eulenspiegel folk book from 1515 goes like this: Till was a brewer in Einbeck. His master went to a wedding and told him to brew the beer in his place, i.e. “boil the hops well”. Till played a trick on him and threw the master's dog (named Hopf) into the hot water instead of the hop plant. When the brewer returned and drained the brew, he found the remains of his dog in the brewing pan.
“The dog in the pan goes crazy” is a nonsense expression used when telling unbelievable stories. And it probably comes from that Till Eulenspiegel episode.