[Brexen]

I go through a list of the follwoing, checking for any errors or unwanted changes;

Login/Logout screen
FORUMHOME
Thread Display
Thread and Posts
Postbit
User Profile
Settings (go through every tab changed)
Control Panels (Admin and Mod)
Registartion
Who's Online, Member list, Calendar etc
etc

I then proceed to check the other modifications like Shoutbox, Activity counter etc to see if there are conflicts.

But I ALWAYS test new Modifications, plugins or anything that involves editting templates on a test forum before adding it on my main forum. Even the smallest of things like Postbits.

When you have checked everything, get another user to do the same checks you have done and try different browsers also.