I've heard that there is a dispute among halachic decisors if one is allowed to use an urn like the one shown below on Shabbos
because the outside water tube gets cold over Shabbos but then gets reheated in the cup. Both sides agree that "Yesh bishul achar bishul" (there is cooking after cooking).
What is the argument? It seems to be a machlokes b'mtzius (an argument over a fact).