A few days ago I made a little flowchart describing how, given plugin X, you could determine if it was conflicting with some other plugin Y, or with your theme.
It seems to have really taken off on twitter, so I’m posting it here as well, for safe keeping:
To the extent possible under law, scribu has waived all copyright and related or neighboring rights to WordPress Plugin Troubleshooting Flowchart. This work is published from: Romania.
There’s also a Japanese version.
It seems I went over the process of figuring out why a plugin is broken before.