At my previous company, one of the devs added some debug output, but they added it to a hot loop and forgot to if it out in production.

As a result, all of our frontend servers slowed to a crawl because the logging calls were thrashing our disk I/O - and keep in mind, this is an application which never hits the disk and lives entirely in memory, mostly just assembling data structures from other APIs and combining them into a response for the client. To see a server like that brought to its knees by disk I/O from three lines of debug logging was... impressive, to say the least.