While the upgrade is running using another browser attempt to browse to the site. Check the page is displayed correctly.
Put the site into maintenance mode and using another browser try to browse the frontpage. (you should get the maintenance page and importantly no errors).
Repeat this using standard and any canvas based theme.
Add the following to your config file:
$CFG->theme = 'clean';
Using your browser:
Perform a fresh installation.
Upgrade a 24 site to 25 and then master.
While the upgrade is running using another browser attempt to browse to the site. Check the page is displayed correctly.
Put the site into maintenance mode and using another browser try to browse the frontpage. (you should get the maintenance page and importantly no errors).
Repeat this using standard and any canvas based theme.
Using the command line interface:
Install a fresh site.
Upgrade a 24 site to 25 and then to master.