An error page can either use the same template as the rest of the site or a different one.
With Error 500 (Server Error), it is preferred for the error page to be as simple as possible to lower the probability of the error page itself causing another Error 500. With other errors, this should not pose a problem.
Other than technical reasons (some technologies might only allow static error pages), only few pros come to mind:
- different layout will emphasise that something is wrong
- redundancies are eliminated
When the error page uses a different layout, it might be less obvious which site the error page comes from, though.
It seems to be less common today, some sites that employed different layout in the past recently switched to a single template.
Are there any other reasons for creating a separate layout for the Error 404 page?