My personal favorite was when my company switched to configuring Jenkins in YAML, with some of the config being in YAML proper and other config being in Groovy

My personal favorite was when my company switched to configuring Jenkins in YAML, with some of the config being in YAML proper and other config being in Groovy embedded inside of multiline strings. Since it's Jenkins, the Groovy itself embeds multiline strings for scripts that need to run, so the languages end up nested three levels deep! The only thing that saves me is IntelliJ's inject-language-in-string feature.