At a startup I worked, there was an engineer whose code was incoherent and buggy. So, we were literally better off if that engineer did nothing because their net output was negative. Engineers like that become weaponized with LLMs, and negative numbers become larger negative numbers when scaled up.
I've seen similar. They wasted weeks of senior engineering time, between reviews, meetings, and follow up in Slack, only to have the PR closed without merge. The offending individual was eventually moved to another project.