When you aren't being forthright with the position you held on a development team. If you were a late onboard to a project and spent your time firefighting bug reports, you could perhaps lend cachet to your resume by omitting the relatively minor role you had in the project. Then you'd be borrowing the "plume in your cap" from your team, who deserves more of the credit.

edit: this is my best-fit guess. I have no idea what I'm talking about.