From Stadtwiki Karlsruhe

K-town, Germany is, contrary to common belief in Karlsruhe, US Army slang not for Karlsruhe but for Kaiserslautern. That name is even more likely to leave the tongue of the average GI in a tight knot than that of the “Badische Residenz”.

A true native of Karlsruhe will dislike people from Kaiserslautern, since their football club, over the decades, more often than not defeated the local Karlsruher SC.