Usually people that I've worked with that have this kind of habit also have other habits, like copying open source into commercial code with no regard for licen

Usually people that I've worked with that have this kind of habit also have other habits, like copying open source into commercial code with no regard for license, checking in code under other people's names, flagrantly disturbing massive chunks of code (e.g. checking in whole-repository reformat) without telling anyone, etc. It's definitely marginal and indicates a kind of thoughtlessness. Firing? Maybe not. But it's not easily forgotten.