0. Ensure debugging is set to DEVELOPER level
1. Start a restore (using any backup file).
2. Choosing to restore to a new course, proceed to Step 3 (Settings).
3. Click Cancel.

The following warning appears:

You should really redirect before you start page output

    line 762 of /lib/outputrenderers.php: call to debugging()

    line 2591 of /lib/weblib.php: call to core_renderer->redirect_message()

    line 239 of /backup/util/ui/base_ui.class.php: call to redirect()

    line 272 of /backup/util/ui/restore_ui.class.php: call to base_ui->cancel_process()

    line 446 of /backup/util/ui/restore_ui_stage.class.php: call to restore_ui->cancel_process()

    line 105 of /backup/util/ui/restore_ui.class.php: call to restore_ui_stage_settings->process()

    line 80 of /backup/restore.php: call to restore_ui->process()

Other than the debugging message this does not cause a problem (the 'Continue' link works correctly as expected).

This probably is something I broke with the backup/restore changes in 2.6. It's broken in current master. I suspect it will be difficult to fix 'properly'; not sure if there is an easy way to simply remove the warning.

(At present I don't really have a lot of time to fix this, seeing it is a trivial problem and doesn't cause practical issues, but thought it might be worth documenting anyhow.)