Their positive impact on projects is best when they're on vacation or spinning their wheels in meetings. Everything they touch could instantly earn an @Deprecated annotation, and often when they're focused on their next tasks, the rest of the team is left to either throw out and rewrite, duplicate in parallel or at best refactor the work they've done.
Some of the best code they write is copy and pasted from other code, some of the worst is of their own design. Often the team works hard to try to minimize the impact of the developer, but they simply just don't understand what they're doing.
Agile projects make the results of this developer even more vicious as they get to choose what area of the project they will molest next. Also, many teams are "agile but" where the team follows some practices, they are not empowered to remove that developer from the team.
Often those that make employment changing decisions don't have the tools to validate such a claim. So what's one to do with a negative velocity developer? How does one capture enough information to prove to someone non-technical that their skill sets would best be used elsewhere?
Note: This is not a current situation for me.